Flow Control March 2014[1]

YES

NO

Is It Time To SWITCH?Key Considerations for Managing Wet Seal to Dry Seal Conversions

SOLUTIONS for FLUID MOVEMENT,MEASUREMENT & CONTAINMENT

Wastewater Monitoring s Coriolis vs. Ultrasonic Flowmeters s FCC Rules On Radar Level Probes

MARCH 2014 Vol. XX, No. 3www.FlowControlNetwork.com

Write in 1 or Request Info Instantly at www.FlowControlNetwork.com/freeinfo

features

contents

16 Wet to Dry Seal ConversionBy Raphal BridonDry gas seals are specied in the majority of new centrifugal

compressors; yet many installed units are still equipped with

conventional oil sealing systems.

22 Using Ultrasonic Level Transmitters for Wastewater Discharge Monitoring

By Peter Ward

Businesses that make discharges into rivers, smaller water-

courses, or the sea are usually required to monitor ow to meet

local requirements and protect the environment and human

health. Accurate measurement of these ows is not only impor-

tant for local compliance, but also, submitting inaccurate data

could signicantly impact a companys operating costs.

26 Seeing Through the SteamBy Amin AlmasiSteam generation systems are critical units in many industrial

and power plants, and the boiler feedwater (BFW) pump plays a

key role in steam generation systems. The BFW pump is a spe-

cial kind of pump that requires careful design and operation.

30 Coriolis vs. Ultrasonic FlowmetersBy Jesse Yoder, Ph.D.

It is interesting to compare Coriolis and ultrasonic owmeters,

as a great deal of new product development is occurring with

both of these meter types. In addition, Coriolis and ultrasonic

represent the two fastest growing owmeter categories, with

the possible exception of multiphase owmeters.

2 | March 2014 Flow Control Magazine

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march 2014 | Vol. XX, No. 3

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16

26

1616

22

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To meet increasing regulatory, environmental, social and bottom-line demands, rely on Thermatel TA2 for your energy management solutions.

UNatural Gas UAir Flow U Flare Gas UDigester, Land ll and Bio Gases

Visit our new portal at magnetrol.com/thermalmass ow for more information on energy management and thermal ow metering technology.


contents continued

departments


8 NEWS & NOTES what do the fccs revised rules for radar level devices mean?; water & wastewater

to drive pump sales in china; us cybersecurity framework

provides how to guide for critical infrastructure

40 UP CLOSEwith Rosedales high-ow ltration system

42 NEW PRODUCTS

45 WEB RESOURCE FILE

46 ADVERTISER/PRODUCT INDEX

47 WORD SEARCH natural gas

48 QUIZ CORNERwhich owmeters measure velocity?

columns6 VIEWPOINT so many ways to follow ow control

14 APPLICATIONS CORNERthe new guy steps out on a troubleshooting mission

34 PUMP GUY life is good (or is it?)

38 AUTOMATION FILEconsidering emersons integrated operations initiative

On the Cover: Background image by Irina Titova/ThinkStock;

seal image courtesy of Dresser-Rand.

38


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letter highlights new technical feature ar-

ticles posted to FlowControl Network.com.

The items featured here will include the

cover stories from each edition of Flow

Control magazine and other articles

that may have been cover-worthy, if

only we had more covers to work with.

When you click through to the article,

youll have the opportunity to down-

load a PDF version of the article as it

appeared in the print edition. This op-

tion aims to make it easy for you to

save articles that are of particular in-

terest for future reference.

If you arent a subscriber to Flow Con-

trols e-newsletters, I encourage you to

sign up now at wctrl.com/fc-enews. As

always, we welcome your feedback on any/

all of the above. Please send your com-

ments and suggestions at the email ad-

dress below. FC

Thanks for your readership,

Matt Migliore, Director of Content

[email protected]

The Weekly

Buzz is a list-

based e-news-

letter, highlighting ve new and notable posts

from the prior week on FlowControlNetwork.

com. Any given edition of this e-newsletter

may showcase interesting news, technical

articles, white papers, videos, job opportuni-

ties, etc. recently added to our website.

The Blog Roll is a bi-weekly e-news-

letter that features recent posts on our

FlowStream Blog. Our blog covers stories

related to our focus on solutions for uid

movement, measurement, and contain-

ment. Our goal with the FlowStream Blog

and our Blog Roll e-newsletter is to shed

light on stories that that make you say to

yourself, Ah, thats interesting.

From the Vault is a monthly e-

newsletter that digs back into the Flow

Control archive to showcase a popular

article from our past. Over the years, weve

published quite a few technical articles and

guest columns that have generated signi-

cant reader feedback These are the ar-

ticles that youre likely to nd featured in

our From the Vault e-newsletter.

Finally, our Premium Content e-news-

VIEWPOINT


So Many Ways to Follow Flow Control

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EXECUTIVE DIRECTOR OF BUSINESS

DEVELOPMENTJOHN P. HARRIS | (205) 408-3765

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EXECUTIVE DIRECTOR OF CONTENT

MATT MIGLIORE | (610) [email protected]

MANAGING EDITOR

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EDITORIAL ADVISORY BOARDLarry Bachus: Bachus Company Inc.

Gary Cornell: Blacoh Fluid Control

Jeff Jennings: Equilibar LLC

Mitch Johnson: JMS Southeast

Peter Kucmas: Elster Instromet

Jim Lauria: Water Technology Executive

James Matson: GE Measurement & Control

John Merrill, PE: EagleBurgmann Industries

Steve Milford: Endress+Hauser U.S.

David W. Spitzer, PE: Spitzer and Boyes LLC

Tom Tschanz: McIlvaine Company

John C. Tverberg: Metals and Materials Consulting Engineers

Jesse Yoder, Ph.D.: Flow Research Inc.

WINNER

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THE OUTLOOK | News & Notes

Previously, the methodology for

measuring radio-frequency emissions

made it virtually impossible for LPRs to

meet the requirements unless they were

contained (primarily used in tanks),

says Cynthia Esher, president of MCAA.

The new rules allow LPRs to be used in

the U.S., virtually anywhere, in a num-

ber of frequency bands.

Prior to this new ruling, level prob-

ing radars had to be licensed as low-

power devices per the rules in FCC Part

15.209, says John Benway, engineer-

ing manager for Magnetrol International

(www.magnetrol.com). The low power

requirements restricted level probing ra-

dars to applications inside tanks.

However, Benway says end-users

are increasingly interested in using the

technology outside tanks. A typical ap-

plication is ow in the water & waste-

water industry. The level in a specially

designed constriction of a ume or weir

can be related to ow. These measure-

ments are made outside. The technol-

ogy generally used today is ultrasonic,

which can be susceptible to environ-

mental conditions, such as tempera-

ture, wind, and humidity, none of which

are an issue for radar technology.

Specically, the order modies Part

15 of the FCC rules for level probing ra-

dars to operate on an unlicensed basis

in the 5.925-7.250 GHz, 24.05-29.00

GHz, and 75-85 GHZ bands, and revises

the measurement procedures to provide

more accurate and repeatable measure-

ment protocols for these devices.

Customers increasingly requested

these products, which are available in

other parts of the world, said Esher.

Obtaining individual waivers to sell in

specic applications or in individual

frequency bands was very difcult and

time consuming, which limited the abil-

ity of manufacturers to meet the needs

of their customers.

The basis for this ruling was brought

to attention of the MCAA staff and

Board of Directors in 2009, and several

companies decided to work together on

the project, reviewing the existing FCC

rules for level probing radar equipment.

Once funding for the project was

received, the group began developing

a proposal for workable rules, which

included procedures for testing devices

for compliance. MCAAs memorandum

on the issue was submitted to the FCC

in early 2011.

It took over a year for the FCC Notice

of Proposed Rulemaking to be issued

as they allowed for comments and re-

sponses from the public. The new rules

were published in January of this year.

The initial press release on the rul-

ing noted that special technical char-

acteristics posed previous obstacles

to FCC approval. Esher touched on this

by saying, The previous FCC rules for

unlicensed devices, including radars,

required low transmitter power and a

relatively uniform signal that lacks ex-

treme peaks of energy. An LPR signal is

inherently peaky and, for that reason,

did not comply. Moreover, the permitted

transmitter power was inadequate for

New FCC Rules On Radar Level Probes Enable LPRs to Operate Virtually Anywhere In the US Without a LicenseThe order modifies Part 15 of the FCC rules for level probing radars to operate on an unlicensed basis in the 5.925-7.250 GHz, 24.05-29.00 GHz, and 75-85 GHZ bands.


By Jake Mastroianni

A recent FCC ruling opens up the op-portunity for radar level devices to be employed for applications outside of tanks, such as open-channel umes. Previously these applications were primarily handled with ultrasonic level measurement devices (pictured here). (Photo courtesy of Magnetrol International.)

The U.S. Federal Communications Commission (FCC, www.fcc.gov) has adopted rules that are geared specifically to level probing radars (LPRs) that allow these devices to operate

anywhere in the country without a license. The Measurement,

Control & Automation Association (MCAA, www.measure.org)

worked with the FCC to provide information to the technical

office within the FCC, which crafted a Notice of Proposed

Rulemaking in 2012. The FCC action amends the existing

rules to account for LPR special technical characteristics,

which posed previous obstacles to FCC approval.

many LPR applications.

The new rules are designed to take

these issues into account in permitting

usefully high transmitter power, while

avoiding signicantly increasing the risk

of interference to other spectrum users.

The ruling harmonizes the require-

ments with European standards, says

Benway. The sister organization to the

MCAA, the Canadian Process Control

Association (CPCA), is working with In-

dustry Canada to have the Canadian

rules harmonized.

Harmonization of the standards in

the U.S., Canada, and Europe means

manufacturers can now supply a single

product that can be sold in each of

these regions. The ruling also standard-

izes and harmonizes the testing meth-

ods. These are major benets for sup-

pliers, says Benway.

The order from the FCC mentions

MCAAs request that the agency con-

tinue to provide an option to certify

LPRs under the general emission limits

of Section 15.209. FCC also agreed to

continue to allow LPRs to operate in any

frequency range that does not contain a

restricted band, as permitted by Section

15.209, since some LPRs need band-

widths wider than the new rules permit

to achieve precision measurements.

While members would have liked to

include other frequency bands, Esher

says The new rules will allow the ma-

jority of manufacturers to provide LPR

instrumentation to customers for use in

a wide range of applications without li-

censing from the FCC.

Jake Mastroianni is the manag-ing editor of Flow Control magazine

and FlowControlNetwork.com. He

can be reached at JMastroianni@

GrandViewMedia.com.

www.owcontrolnetwork.com March 2014 | 9

International Biomass

Conference & Expo

March 24-26, 2014

Orlando, Fla.

www.biomassconference.com

Offshore Technology Con-

ference

May 5-8, 2014

Houston, Texas

otcnet.org/2014/

International Fuel Ethanol

Workshop & Expo

June 9-12, 2014

Indianapolis, Ind.

www.fuelethanolworkshop.com

National Advanced Biofuels

Conference & Expo

Oct. 13-15, 2014

Minneapolis, Minn.

www.advancedbiofuelsconfer-

ence.com

Upcoming Events:


trendlines



Water & Wastewater to Drive Pump Sales In China to $8.5B in 2014

Global Valve Market to Reach $82.5B By 2017

Gas Turbine and Combined Cycle Power Plants Bolster Uptake for Cross-Flow Membrane Systems

Pump sales in China will reach $8.5 billion in 2014, according to Pumps

World Market report by

McIlvaine Company (www.

mcilvainecompany.com). Over

$3 billion will be spent by

municipalities for wastewater

and drinking water plants.

The continuing popula-

tion shift to cities, the higher

standard of living, and in-

creased expectations are

driving infrastructure expan-

sion, the report says.

The report says the pump

expenditures by power plants will ex-

ceed $1.2 billion, and there will be an

addition of more than 50,000 MW of

new power plants this year.

In addition, there will be some retro-

ts of ue gas desulfuriza-

tion (FGD) systems. These

systems have large pump

requirements. A big effort

to reduce NOx is resulting

in a market for ammonia

pumps, the report says.

The chemical industry

is growing thanks to both

international and domes-

tic investment.

The report says mines

will spend over $600 mil-

lion for pumps for use in

China. Chinese compa-

nies are also inuencing

pump purchases in Africa and South

America.

Global demand for industrial valves is forecasted to rise 5.1 percent a year through 2017 to $82.5 billion, according

to World Industrial Valves by The Freedonia

Group Inc. (www.freedoniagroup.com).

Although growth is expected to be healthy

across the globe, the drivers of growth

will vary by region. Advances in develop-

ing areas, such as China and India, will

result from ongoing industrialization, as

investment in water infrastructure and

electricity generation grows, says Michael

Deneen, Freedonia analyst. In developed

areas, continued advances in manufactur-

ing output are expected to provide growth

in the process manufacturing market. Oil

producing nations, such as those in the

Middle East, will see gains due to rising

production, according to the report. In the

U.S., demand in the oil and gas market

is expected to benet from infrastructure

construction and increased production due

to shale development, as well as from the

improved economy.

The market for cross-ow membrane systems, repair parts, replacement membranes, chemicals, and related instru-mentation for use in gas turbine and combined cycle power

plants will exceed $550 million in 2014, according to McIlvaine

Company (www.mcilvainecompany.com). Gas turbine systems

utilize cross-ow systems, including reverse osmosis, ultraltra-

tion, and microltration, in a number of processes.

Applications include:

1 Intake water

2 Boiler feedwater

3 Cooling tower recycle

4 Fogging nozzle dematerialized water

5 Wastewater from the cooling tower and balance of plant.

iStock/ThinkStock

In the February 2014 issue of

Flow Control, we incorrectly noted

the website address for the Con-

trol System Integrators Association

(page SS-7). The correct website

address is www.controlsys.org.

Help us make Flow Control the

best it can be! If you see any

errors, mix-ups, or oversights,

whether grammatical or techni-

cal, please email JMastroianni@

GrandViewMedia.com.

accountability file

best practices & standards

www.owcontrolnetwork.com March 2014 | 11Write in 8 or Request Info Instantly at www.FlowControlNetwork.com/freeinfo

EPA Revises Guidelines for the Use of Diesel Fuels In Hydraulic Fracturing

The U.S. Environmental Protection Agency (EPA, www.epa.gov) released revised underground injection control

(UIC) program permitting guidance for

wells that use diesel fuels during hydrau-

lic fracturing activities.

The EPA developed the guidance to

help clarify how companies can comply

with a law passed by Congress in 2005,

which exempted hydraulic fracturing op-

erations from the requirement to obtain

a UIC permit, except in cases where die-

sel fuel is used as a fracturing uid.

The EPA is issuing the guidance

alongside an interpretive memorandum,

which claries that class II UIC require-

ments apply to hydraulic frac-

turing activities using diesel

fuels, and denes the statu-

tory term diesel fuel by refer-

ence to ve chemical abstract

services registry numbers.

For EPA permit writers,

the guidance outlines exist-

ing class II requirements for

diesel fuels used for hydraulic

fracturing wells and technical

recommendations for permitting those

wells consistently with these require-

ments.

The EPA says decisions about per-

mitting hydraulic fracturing operations

that use diesel fuels will be made on

a case-by-case basis, considering the

facts and circumstances of the specic

injection activity and applicable stat-

utes, regulations and case law.

iStock/ThinkStock

Valves, Actuators & Controls 101 Course Set for April 2-3 in Kansas City

The Valve Manufacturing Associations (VMA, www.vma.org) next Valves, Actuators & Controls 101 course will

take place April 2-3 at the Hilton

Kansas City Airport in Kansas City, Mo.

The seminar is designed to give profes-

sionals and distributors a better under-

standing of modern valves, actuators,

and controls. The Valves, Actuators &

Controls 101 course consists of lessons

that aim to guide attendees through the

world of valves and the systems they

help control, starting with the simplest

types of valves and moving into the

more complicated automated products,

including actuators and controls. The

course has been expanded to two full

days, with the addition of two new les-

sonsSolenoids and Limit Switches,

and Positioners and Other Actuator

Accessories.



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ROSEDALEHIGH FLOW HOUSINGS1 to 18 Filter Elements in 40 & 60 Lengths!

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US Cybersecurity Framework Aims to Provide How-To Guide for Critical Infrastructure

The Obama Administrations Cybersecurity Framework is the result of a year-long private-sector led effort to develop a voluntary how-to guide for organizations in the critical infrastruc-

ture community to enhance their cybersecurity. The framework

is a deliverable from the Executive Order on Improving Critical

Infrastructure Cybersecurity that President Obama announced

in the 2013 State of the Union. Through the development of

this framework, industry and government aim to strengthen

the security and resiliency of critical infrastructure in a model

of public-private cooperation. Over the past year, individu-

als and organizations have provided their thoughts on the

kinds of standards, best practices, and guidelines that would

meaningfully improve critical infrastructure cybersecurity. The

Department of Commerces National Institute of Standards

and Technology (NIST, www.nist.gov) consolidated that input

into the voluntary Cybersecurity Framework.

best practices & standards

Fieldbus Foundation Postpones 2014 General Assembly in Rotterdam Due to Security Restrictions

The Fieldbus Foundation (www.eldbus.org) has postponed its 2014 General Assembly in Rotterdam due to trav-el and security restrictions associated with the upcoming

Global Nuclear Security Summit (NSS) in The Hague, The

Netherlands. Originally planned for March 25-28, the Fieldbus

Foundations global FOUNDATION eldbus end-user seminar

will be rescheduled for the fall of this year somewhere in

Europe (location pending), while its End User Advisory Council

(EUAC) meeting and business meeting will be conducted elec-

tronically on the original dates of March 26 and March 28,

respectively. The EUAC session will be the rst in a series of

such meetings to be held electronically this year.


APPLICATIONS CORNER | Process Instrumentation

Last month, I alluded to a complicated problem at a plant where I was a new hire (Flow Control, February, page 12). My recollection is that the issue surfaced during the second week that I worked in the plant when my

immediate supervisor and my (only) electrical/instrumenta-

tion foreman were both coincidently on vacation, so I carried

the radio.

It was a warm day in June when I was

called out to the unit at about 2 p.m. after

a unit shutdown because the ow of one

of the reactor feeds was low. New to the

plant and on my own, I had to rely on my

knowledge of basics, such as understand-

ing Process and Instrumentation Drawings

(P&ID), loop drawings, heat and material

balances, operating procedures, safety procedures, hazard

reviews, and pump curves to resolve the problem.

When I arrived at the unit, I found the owmeter installed

in a submerged piping leg at approximately waist height

above grade. Its corresponding control valve was located

downstream and was almost entirely covered with ice. The

midget-maker was open to allow access to the owmeter

electronics.

What is a midget-maker? you might ask. Well, cabinets

typically open to the left or to the right, depending upon the

location of their hinges. In this installation, the cabinet was

installed with its hinges on the top so it opened upwards

and was held open by a precarious-looking metal brace. The

technicians referred to this cabinet as the midget-maker

because if the metal brace became dislodged while a techni-

cian was working on the owmeter, its door would fall onto

the technician and make him shorterhence the midget-

maker nickname.

I probably dont have to say (but I will anyway) cabinet

hinges should be located on the left or right; not on the top or

bottom. More next month FC

Back to BasicsThe New Guy steps out on a troubleshooting mission

Flow Problems?>,*(5/,37>> Troubleshoot Problem Flow Measurements

>> Assess Billing Flowmeter Accuracy

>> Close Plant and Water Balances

>> Improve Flowmeter Accuracy

>> Select and Specify Flowmeters

>> Train Your People

WE KNOW FLOW!

XXXTQJU[FSBOECPZFTDPNtWrite in 12 or Request Info Instantly at www.FlowControlNetwork.com/freeinfo

David W. Spitzer

David W. Spitzer | Flowmeter | Instrumentation |

Process Measurement | Reactor

FLOWSTREAM Find related content @ owcontrolnetwork.com

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC

offering engineering, seminars, strategic, marketing con-

sulting, distribution consulting and expert witness services

for manufacturing and automation companies. Spitzer and

Boyes is also the publisher of the Industrial Automation

Insider. David has more than 35 years of experience and

has written over 10 books and 300 articles about flow

measurement, instrumentation and process control.

David can be reached at 845 623-1830 or www.spitzerand

boyes.com. Click on the Products tab to find his Consumer

Guides to various flow and level measurement technologies.

It was a warm day in June when I was called out to the unit at about 2 p.m. after a unit shutdown because the flow of one of the reactor feeds was low.

On-spec. Not off-the-shelf.While some owmeter companies offer off-the-shelf solutions that may or may not work for you, Hoffer Flow Controls engineers and builds

owmeters for your specic purpose, regardless of what that purpose requires. We meet the critical demands of even the most challenging applications.

And we know that the best owmetering system is often the solution designed specically for it. Regardless of your application, you can count on Hoffer

to engineer a precisely accurate, reliable, easy-to-implement owmetering system. Purpose-built. No matter what your purpose.

,((( #+ (

Purpose-Built


In addition, the following precautions

should be taken during project execu-

tion to ensure successful conversion:

perform a detailed physical integration

analysis of the dry gas seal in the exist-

ing compressor; conduct a detailed ro-

tor dynamic analysis; select the proper

gas seal system design for the compres-

sor; and plan for operator training.

This article will discuss the factors

end-users should consider before up-

grading to dry gas seals, and the steps

that should be taken to ensure a suc-

cessful conversion once the decision is

made to retrot a compressor with dry

gas seals.

Dry Gas SealsTo expect a totally leak-free sealing

system between two parts in relative

movement is unrealistic (e.g., between

a static and a rotating part; between a

housing and a shaft; in pumps, ther-

mal motors, etc.). There are, however,

efficient devices that may limit leaks,

friction, and wearing at the interface of

the moving parts.

Gas seals are among the most ef-

cient means to minimize process gas

leakage to the atmosphere and to re-

duce wear and friction.

The gas seal is also a reliable means

to route efuent leaks to safe areas. Over-

all, the whole gas compression process

benets from the dry gas seal system.

Figure 1 shows the location of the

seals in a typical centrifugal compres-

sor. Their location is quite strategic, as

they are the interface between the in-

side of the compressor (gas process at

high pressure, high temperature) and

the atmosphere (air and oil mist from

the bearing cavity).

Due to the balance line, the gas seal

only has to deal with the intake pressure

of the compressor.

As will be explained later, the gas seal

requires a high-quality gas to operate.

Therefore, instead of using the gas pres-

ent in the balance line, the seals are fed

with a clean and dry gas, typically taken

at the discharge of the compressor.

This gas is dried, ltered, heated (if

necessary), and its pressure lowered to

slightly above the intake pressure be-

fore being injected at the primary port

of the seal.

The gas-seal principle is simple (Fig-

ure 2). The leakage (process gas) must

be routed to a safe area; therefore, the

leakage is forced to pass between a

static and a rotating part. The rotating

part is a grooved ring driven by the com-

MAINTENANCE & RELIABILITY | Sealing Systems


Wet Seal to Dry Seal CONVERSIONConsidering the benefits of retrofitting your compressor

Dry gas seals are specified in the majority of new centrifugal compressors; yet many installed units are still equipped with conventional oil sealing systems. The benefits of dry gas

seals are such that conversions from traditional oil seals to

dry gas seals may be advantageous to compressor operators.

However, end-users should ask themselves several questions

before deciding to retrofit their compressors with dry gas seals.

The decision to retrofit a compressor with these upgraded

seals may be dictated by economic factors, HSE constraints,

or technical considerations. Users should consider all of these

factors when deciding whether or not to upgrade a compressor

with dry gas seals.

By Raphal Bridon

Figure 1. Centrifugal compressor cut-away


pressor shaft. The static part is a ring

facing the rotating ring (but with only

light axial movement).

When rotating, the grooves generate

an aerodynamic effect that creates a

gap (from 4 microns to 10 microns) be-

tween the rotating and stationary rings.

The ow generated by the pressure dif-

ferential leaks between the two faces,

and then this gas leakage is routed

to the venting system of the machine

(ared) or vented.

Because of the gas lm between the

faces, this constant gap between them

prevents the parts from rubbing against

each other and makes the gas seal a

contact-free device.

Gas Seal Arrangements A tandem gas seal is typically

used for non-hazardous gases. In

this arrangement, the sealing gas is in-

jected at a pressure slightly above the

intake pressure, so that a vast major-

ity (over 80 percent) of it passes under

the inner labyrinth teeth. The remainder

(less than 20 percent) passes through

the gap created by the lift-off effect and

leaks to the are (18 percent). The last

sealing gas residues (2 percent) leak

through the secondary stage to the vent.

The other important device in the

compressor seal is the tertiary (or sepa-

ration) seal, which may be a labyrinth or

segmented carbon rings. Its function is

to prevent the bearing oil mist from mi-

grating to the seal and the sealing gas

from migrating to the bearing oil. This

separation is made by a gas leak, which

prevents the oil from entering the gas

Figure 2. Cutaway and cross-section of a simplied gas seal.


PLASTIC CONTROL VALVES FORALL YOUR CORROSIVE APPLICATIONS

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seal area on the inboard side, and also

prevents the sealing gas coming from

the secondary stage of the seal from

polluting the bearing oil.

So, depending on the nature of the

separation gas, the gas seal vent may

vent a mixture of sealing gas (hydrocar-

bon) and nitrogen, or a mixture of seal-

ing gas and air.

A tandem gas seal with an in-

termediate labyrinth is used when

the process gas is hazardous, e.g.,

lethal gas, ammable gas, or when

it condensates at the primary seal

outlet (Figure 3). In this scenario, a

buffer gas, such as nitrogen, sweet gas

or fuel gas, is required.

The principle is the same as in the

tandem gas seal with the addition of an

intermediate labyrinth fed with an in-

termediate (sometimes called a buffer)

gas, generally nitrogen. This prevents

hazardous seal gas from leaking into the

atmosphere.

A double (-opposed or back-to-

back) gas seal is used when the pro-

cess gas is dirty, or when the seal-

ing pressure is close to atmospheric

pressure. A sealing gas (typically aux-

iliary gas) is needed, such as nitrogen,

sweet gas or fuel gas.

The conguration consists of two

sealing faces (rotating ring and static

seats) in a back-to-back arrangement.

A primary advantage of this seal type is

the lower number of ports required

one for the sealing gas; one for the

vent; one for the separation gas; and

one for the buffer gas (optional).

Because of the pressure differential

between the inboard side of the gas seal

and the sealing-gas port, and between

the vent and the sealing-gas port, the

ow is not symmetrical (a majority of

the sealing gas enters the machine).

Generally in low-pressure applica-

tions, the available process gas pres-

sure is not suitable to feed the gas seal,

so an alternate source must be consid-

ered (e.g., nitrogen, fuel gas).

The nature of the sealing gas must

also be compatible with the nature of

the process; the alternate source could

trigger unwanted chemical reactions or

damage the downstream catalyst.

Why Convert Wet Seals to Dry Gas Seals?

1The number one reason for

retrofitting conventional wet

seals to dry gas seals is reliability.

Dry gas seals are non-contacting

mechanical seals, which eliminates the

issue of seal wear. Theoretical lifetime

is limited only by the secondary sealing

elements (usually o-rings or polymer-

based seals) whose lifespan can be as

long as 15 years. It is not uncommon to

see dry gas seals operating for more

than 10 years before being refurbished,

which is much longer than is expected

for oil seals.

Not only is the seal itself more re-

liable; so is the whole sealing system,

because it is made of static compo-

nents. Oil seal systems, on the other

hand, have more components, includ-

ing rotating machines (pumps, motors/

turbines) and are more often prone to

unscheduled maintenance.

2Local (or company-wide) HSE

Regulations: Elimination of oil

contamination by process gas has a

positive environmental impact, since

sour oil needs to be treated, stored,

and disposed of. Sour seal oil treat-

ment and disposal also has a signifi-

cant cost.

In terms of safety, disposing of con-

taminated oil removes a hazard of ex-

plosive mixtures in the oil reservoir of

seal (and lube) systems.

3Reduced Operating Costs:

Energy costs drop significantly,

since seal oil pumps and degassing

tank heating systems are not required

when using dry gas seals. Power losses

due to shear forces in gas seals are

much lower than losses experienced in

oil seals, which results in energy sav-

ings as well.


Figure 3. Tandem gas seal with intermediate labyrinth

Dry gas seals have several advantages compared to conventional wet seals: higher reliability; safer operation; reduced emissions; lower operation and maintenance costs; and improved process gas quality. These advantages may help end-users justify an investment if an acceptable return on investment can be demonstrated.

4Reduced Maintenance Costs:

As stated above, the simplicity of

gas seal systems means routine main-

tenance is less frequent and less costly

than it is with oil seal systems.

5Reduced Emissions: Wet seal gas

leakages are reduced 10-fold with

gas seals, credited to the very thin running

gaps between the seal faces. This results

in cost savings for the end-user and

reduced penalties on taxable gas flaring.

6Process Quality: Contamination

of process gas by seal oil is

eliminated, enabling higher quality pro-

cess gas. Costs related to oil removal

from process gas are also eliminated. A

good example is closed loop/refrigera-

tion processes where process gas treat-

ment is costly.

7Maintainability: Some opera-

tors now have more experience

with dry gas seals than with oil seals.

This may compel end-users to retrofit a

fleet at a specific plant or site to

achieve consistent sealing technology

throughout.

Dry gas seals are supplied as car-

tridges by vendors, and the gas seal

OEM usually performs their mainte-

nance/refurbishment.

These seven benets may not be ap-

plicable to all situations, and it should be

noted that wet seals to dry gas seals con-

versions are not straightforward. The fol-

lowing recommendations are offered to

help make the retrot project a success.

How to Ensure a Successful Retrofit from Wet Seals to Dry Gas SealsPhysical Integration: Integration of the

dry gas seals in the original compres-

sor head/cavity must be checked.

The number and location of supply

and vent ports (at least four ports

are required on gas seals) should be

reviewed. End-users should also con-

sider inboard and outboard diameters;

seal cartridge length; and the locking

system of the gas seal to the com-

pressor shaft.


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In some instances, compressor shaft and compressor head

rework are required. This should be assessed as soon as possi-

ble during the project to avoid project delays and cost overruns.

Seal Systems Study: While dry gas seals operation is usu-

ally not a concern during normal running conditions, tran-

sient conditions (start-up including first start, shutdown) and

standby (pressurized and unpressurized) must be taken into

account during the seal system design. In other words, a

supply of dry and filtered seal gas at the right pressure must

be ensured at all times.

The use of an alternate source of seal gas may be required

during start-up, shutdown, and standby. If not available on

site, end-users may consider supplying a conditioning skid.

This can include a pre-lter, booster, and heater.

In any case, the best way to select the proper source of

seal gas is to run a phase map analysis and make sure that

a sufcient margin (20 C per API 614) to the dew point line

(and hydrates formation line, if applicable) is maintained at all

times in the gas seal panel and inside the gas seal.

On top of the suitability of seal gas, availability and suit-

ability of secondary seal gas (when tandem gas seals with

intermediate labyrinth are selected) and separation gas (usu-

ally nitrogen or air for separation barrier seals or labyrinth)

must be checked.

Finally, a physical integration study of the gas seal panel

must be performed, including space requirements and inter-

connecting piping/tubing to and from the compressor.

Rotor Dynamic Check:

Retrotting from wet seals

to dry gas seals will affect

rotor dynamic response

since oil seals have bet-

ter damping characteristics

than dry gas seals. Perform-

ing a rotor dynamic analysis

will conrm if amplication

factor and logarithmic dec-

rement are still acceptable

with gas seals. In most cas-

es, no further modication is

required; however, there are

some critical applications

(long shaft, high speeds,

etc.) where additional up-

grades must be incorporat-

ed (damper bearings, hole

pattern seals, etc.).

Training: Training should

be standard practice. While

dry gas seals usually require

little monitoring, they are considered black boxes. There are

a few indicators that can help assess the health of a gas seal.

Proper assembly and disassembly (in and from the com-

pressor) is also of prime importance. Failing to do so may lead

to premature dry gas seal failures.

Weighing the AdvantagesDry gas seals have several advantages compared to conven-

tional wet seals: higher reliability; safer operation; reduced

emissions; lower operation and maintenance costs; and

improved process gas quality. These advantages may help

end-users justify an investment if an acceptable return on

investment can be demonstrated.

However, as described in the second part of this article, a

careful review of the system and its operating conditions is re-

quired. Provided all precautions are taken, dry gas seals may well

be the most reliable mechanical seals currently available. FC

www.dresser-rand.com

Raphal Bridon began his career with Dresser-Rand in 1999 as a Technical Support Engineer. He then moved to

a Project Development Engineer position for reciprocating

compressors before working as a Key Account Purchaser

in the aeronautic business. He returned to Dresser-Rand

in 2007 as the Manager for the Gas Seals and Bearings

Business Unit. Mr. Bridon earned his masters degree from

Ecole Centrale Nantes (France).


Find related content @ owcontrolnetwork.comSearch on: FLOWSTREAMCompressor | Maintenance | Mechanical Seal | Reliability

Accurately Measuring FlowOpen channels are widely used by

industrial companies for the discharge

of wastewater.

The most common method of mea-

suring ow through an open channel is

to measure the height or head of the

liquid as it passes over an obstruc-

tionfor example, a ume or weir in the

channel. There is a specic relationship

between the height of inlet water and

the owrate for every open channel that

is free owing through a specic con-

trolled metering structure. This means

that for any given inlet height there will

be a corresponding ow. By plotting this

relationship, the ow can be determined

by accurately measuring the water level

or head using a level sensor.

To satisfy the needs of regulatory

bodies for continuous, accurate, and

reliable ow data, electronic measuring

devices are being increasingly specied

for the measurement of water level.

These have become the instrument of

choice as they provide higher accu-

racy, improved reliability, and reduced

maintenance. Level sensors based on

ultrasonic technology meet these re-

quirements and are being increasingly

deployed for the measurement of ow

in open channels.

Uncertainty In Measurement SystemsIn open channel applications, even with

the best equipment and robust main-

tenance regimes, regulatory authorities

accept that there will be an uncertainty

of measurement. The regulatory author-

ity typically establishes uncertainty lim-

its. For example, in the UK the target is

+/- 8 percent uncertainty for the total

daily volume of effluent discharged.

To achieve this gure, consideration

must be given to all components of the

ow measurement system, including

the manufacture and installation of the

primary devices (umes and weirs) to

the relevant standards, installation, and

commissioning of level measurement

devices and the ow calculation.

Inaccuracies in an open-channel

ow system can be caused by calibra-

tion faults, incorrect installation, incor-

rect construction, super critical ow in

weirs, subcritical ow in umes, oating

debris, environmental conditions, poor

TECHNOLOGY SPOTLIGHT | Water & Wastewater


WASTEWATER Discharge MonitoringUsing ultrasonic level transmitters for open-channel flows

Businesses that make discharges into rivers, smaller watercourses, and the sea are usually required to moni-tor flow to meet local requirements and protect the environ-

ment and human health. Accurate measurement of these

flows is not only important for local compliancesince

most wastewater treatment companies base their charges

on volumetric dischargesubmitting inaccurate data could

have a significant impact on a companys operating costs.

By Peter Ward

Large Parshall ume


computational methods, and the inability of the measure-

ment system to respond to rapidly changing owrates. In

open-channel installations, measurement errors can intro-

duce signicant inaccuracies and these errors can be cat-

egorized into either systematic or random errors.

Systematic errors are repetitive errors that are repeated

in each measurement. These can be caused by a calibration

error or incorrect ume dimensions, etc. Once identied,

these errors can usually be eliminated or reduced.

Random errors are much more difcult to identify. They

are caused by unpredictable or random eventsfor exam-

ple, debris underneath the sensor or the blockage of a V-

notch.

In open-channel ow measurement, most random errors

directly affect the level in the channel or weir, which can

have a considerable effect on the total accuracy. A good un-

derstanding of primary device construction and installation,

and good housekeeping practices will help to keep these

errors to a minimum.

Flumes are available in all shapes and sizes and must be

dimensionally compliantusually in line with an appropriate

standardfor example, BS or ISO.

Maintenance is a major consideration, and cleanli-

ness is important in achieving accurate ow data. If sedi-

ment or bio growth forms on the sides of the approach

channel and ume cheeks, the owmeter will invariably

read high. This would have a signicant impact on indus-

trial dischargers who usually pay for the volumetric dis-

charge based on cubic meters or per 100 gallons. A high

ow reading means excessive charges for the business.

The Importance of Accurate Level MeasurementWhen determining the uncertainty of flow measurement in

an open-channel application, the measurement that has

the greatest effect is the upstream level measured in the

approach channel. It is therefore very important that the

level measurement device used is accurate, reliable, and

V-notch weir on the discharge from small wastewater treatment works


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Screw Gear FlowmeterFor High Viscosity Liquids

Multi-Range Multi-GasFlowmeters and Controllers

unaffected by environmental changes, such as temperature.

The latest generation ultrasonic level transmitters have

been developed specically for use in open-channel ow

measurement and provide highly accurate results. The ultra-

sonic sensor is mounted above the ow stream and transmits

a sound pulse that is reected by the surface of the liquid.

The time required for a pulse to travel from the transmitter to

the liquid surface and back to the receiver is used to deter-

mine the liquid level.

However, since the speed of sound in air varies with its

temperature, it is necessary to compensate for factors such

as the weather and heat gain from sunlight. For example, a

change in air temperature from 20 C to 22 C would translate

into a 1.2 cm (~0.5) change in distance to surface value.

Depending on the size and shape of the channel, this error

could have a signicant effect on calculated volume ow.

To overcome this problem, some modern ultrasonic trans-

mitters are capable of automatically compensating using a

remote temperature sensor connected directly into the trans-

mitter. This is positioned above the liquid surface to accu-

rately monitor air temperature.

The accuracy of ow measurements can also be affected

by variations in surface level caused by surface turbulence or

a sudden variation in ow that cause wave effects. In order

to take account of these changing conditions, a rapid ultra-

sonic pulse rate is required. For example, the ultrasonic pulse

rate of certain modern ultrasonic transmitters is 1 per second

(user congurable 0.5 to 2 seconds), enabling the tracking of

changing ow proles as they occur.

The resolution of the measuring device is also very im-

portant in open-channel applications. Resolution denes the

systems ability to detect and respond to small changes in the

open channel level. For example, a level error of just a few

mm in the inlet height could result in a signicant totalized

ow error. A transmitter with a resolution of 1 mm ensures

that errors due to small changes in channel height are mini-

mized.

To conrm the accuracy of the results provided by the level

transmitter, it is good practice to install a reference target for

the transmitter. This provides both a rigid structure to mount

the instrument and incorporates a retractable target plate

to produce a reference reading at a pre-determined height

above the datum level. This is usually set during commission-

ing using an optical or laser level, which enables the end user

to perform easy checks on the system to ensure the readings

are correct.

Data RecordingTo convert the level measurements into flow, the digital data

from the level transmitter is sent to a flow logger. This com-

bines the characteristics of the flume or weir with the level

data and performs the on-board flow calculation and inte-

grates flow to provide cumulative and daily totals so that dis-

chargers can be charged based on their actual flow volumes.


TECHNOLOGY SPOTLIGHT | Water & Wastewater

Ultrasonic level transmitter with reference reector

Regular inspections ensure systems comply with national standards.

For example, some modern ultrasonic level transmitters

will accept a 4-20mA or HART transmitter input. They can be

pre-programmed with standard volumetric and ow equations

to convert a level signal into contents or ow.

Regulatory authorities usually require historical data of

ow volumes, so the ultrasonic level transmitter should be

able to store ow data in a form that can be downloaded as

an ASCII le, saved as a CSV le on a PC, and imported into

Excel for generating custom reports.

Meeting the Need for Continuous MonitoringTo meet the regulatory requirements for emissions monitoring

of discharges, many companies must continuously monitor

their emissions to meet quality requirements.

The most common method of measuring ow through an

open channel is to measure the height (or head) of the liquid

as it passes over an obstruction. The requirement for accu-

rate and reliable continuous measuring devices has seen the

growth in the use of ultrasonic level transmitters for emissions

monitoring applications.

Ultrasonic level transmitters do not require any mainte-

nance and they have been developed specically for use in

open-channel ow measurement. Temperature sensors are

used to compensate for changes in air temperature. When

used with a ow logger, ultrasonic level transmitter can be

employed to provide historical data in a format that can be

used to conrm compliance with local requirements. FC

www.mobrey.com


Level Measurement | Open-Channel Flow | Ultrasonic Level Transmitter | Wastewater

Find related content @ owcontrolnetwork.comSearch on:FLOWSTREAM

Peter Ward is a senior product manager for Ultrasonic Level Instrumentation with Mobrey Measurement, Emerson

Process Management, based in Slough, UK. Prior to

Mobrey, he worked for GEC Elliott Process Instruments

involved with design of level and flow control systems, and

later Arkon Instruments, Ltd., specializing in open-channel

flow. Mr. Ward has more than 35 years experience in the

Water & Wastewater industry and has been an MCERTS

Inspector (Monitoring Certification Scheme) for flow mea-

surement on behalf of the UK Environment Agency.


/2:(6767$%/()/2:&$3$%,/,7

BFW pumps are used for feed-

ing water to a boiler, where heat

energy will be supplied and feed-

water will be changed into steam

under pressure. Nowadays, BFW

pumps commonly operated at

temperatures of 120260 C (or

more), operating pressures of 50

250 Barg (or above), and power

ranges of 19 MW. High-speed

pumps of 3,000 RPM or more are

normally employed in BFW servic-

es. Due to the extreme conditions

in which BFW pumps operate, they

are prone to failure when improp-

erly designed or operated and are a ma-

jor cause of steam system unavailabil-

ity. Specifying, purchasing, installation,

commissioning, operation, and reliability

improvements on BFW pumps require a

deep knowledge and experience of their

hydraulic, process, thermal, mechanical,

and dynamic behaviors.

BFW Pumps BFW pumps for steam boilers should be

capable of withstanding severe thermal

shocks in order to protect the boiler

since a boiler failure would result in an

unacceptable and costly plant shut-

down due to steam unavailability or

damage.

For safe and reliable operation, the

boiler should have an uninterrupted

supply of feedwater, which is within

close temperature limits of the rated

temperature to avoid thermal shock and

possible damage or even catastrophic

failure. It is therefore essential that a

standby BFW pump be available at all

times. The standby BFW pump should

be capable of accepting within a few

seconds the full ow of full-temperature

water irrespective of its standing tem-

perature, which may be 100-150 C low-

er than that of the running pump. The

major factor in the mechanical design

of a BFW pump is the thermal shock

to which the pump may be subjected.

In addition to the stresses imposed by

pressure and by operation, consider-

ation should therefore be given to the

stresses due to differential thermal ex-

pansion during rapid changes of tem-

perature.

The shell of the barrel of a BFW

pump is usually exposed to severe tem-

perature uctuations on its inner wall,

while its outer wall, usually exposed to

atmosphere, would tend to lag in tem-

perature behind the inner walls

during temperature changes.

This makes BFW pumps prone

to temperature stress within the

metal due to the differential ex-

pansion between the inner and

outer walls.

The thermal shock can cause

risk of internal and external mis-

alignments if the elements of the

pump are allowed to expand un-

equally with respect to the axis of

the shaft. Internal misalignments

could be because of misalignment

of bearings and internal parts of

pumps. External misalignments

are the misalignment of pump, gear unit

(if used), and driver. Stresses can gener-

ally cause risk of misalignment if they are

unsymmetrical around the shaft axis.

The symmetric design is the key for

reliability and safety of BFW pumps. Gen-

erally, in modern turbo-machine designs

particular for extreme temperatures (high

or low temperatures) and high pressure

applications, symmetry about the shaft

axis is an important consideration. The

symmetry of construction, of ow and

of stresses in order to withstand rapid

temperature changes and high pres-

sures without distortion or misalignment

issues, should always be respected for

BFW pump design and selection. It is es-

sential that all thermal ow during tran-

sient conditions, all water ow, all pres-

INSTALLATION GUIDELINES | Boiler Feedwater Pumps


Seeing Through THE STEAMConsidering the critical role of boiler feedwater pumps in steam generation systems

Steam generation systems are critical units in many indus-trial and power plants, and the boiler feedwater (BFW) pump plays a key role in the operation of these systems. The

BFW pump is a special kind of pump that requires careful

design and operation.

By Amin Almasi

An installation of a boiler feedwater pump


sure containing sections, and all stresses

should be as symmetrical as possible

about the shaft axis.

BFW pumps are most often under

rapid heating and relatively slow cooling.

The heating rate could be around 400

550 C per minute and the cooling rate

would be around 4070 C per minute.

The thickness of the high-pressure BFW

pump casings could be 20100 mm,

which is usually the thickest in pumps.

In most severe thermal shock situa-

tions, there is a possibility that stresses

at 515 percent of the casing thick-

ness pass the allowable stress limit,

particularly for large and high-pressure

pumps at high heating-up rates. There

is often no danger in this case, since

the frequency of such shock is relatively

small with respect to the fatigue range

of the material, and since the reversal

shock of cooling is much less severe.

Based on thermal and stress studies for

BFW pump casings, a high tensile steel

should be used. The resulting reductions

of the casing thickness had the advan-

tage of a very much lower temperature

differential between walls, and since the

material had a higher yield point and

consequently higher allowable stresses.

In these designs, maximum experienced

stresses would not pass the limits. An

improvement of corrosion and erosion

resistance could also be obtained with

the higher tensile steels.

The bolts holding the casing ele-

ments together to form a pressure con-

taining system are partially exposed to

the air, and consequently will change

temperature less rapidly than the casing

wall, which is in contact with the BFW.

When the pump is heated suddenly, the

bolt stress is increased by the differen-

tial expansion between the hot casing

and the cool bolts. During this heating

period, the stresses in the bolts should

not exceed the allowable stresses (al-

lowable stresses should be dened

based on yield stresses of the materials

with sufcient safety factors). Converse-

ly, when the pump casing is cooled to

a temperature below that of the bolts,

the resulting differential expansion will

cause a reduction of bolt tension. It is

essential that at their minimum stress

conditions, the bolts contain a suf-

cient margin of tension to hold the main

joints against risk of leakage.

Corrosion & ErosionThe BFW should be specially treated to

avoid problems in the boiler and down-

stream systems. Untreated boiler feed-

water can cause corrosion and fouling.

Corrosive compounds, especially O2 and

CO2, should be removed, usually by the

use of a deaerator.

Deposits reduce the heat transfer

in the boiler, reduce the owrate, and

eventually block boiler tubes. Any non-

volatile salts and minerals that remain

when the BFW is evaporated should be

removed, as they can become concen-

trated in the liquid phase and require

excessive blowdown (draining) to pre-

vent the formation of solid precipitates.

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22069 Van Buren StreetGrand Terrace, CA 92313 USAO: +1 (909) [email protected]


FLOW UPDATE continued

Even worse are minerals that form scale.

The treatment of feedwater to give minimum corrosion

and scale formation of the boiler may result in a liquid that

is quite erosive at the ow speeds associated with a high-

pressure (high-speed) pump.

In other words, the BFW treatment is usually designed to

give minimum corrosion at the boiler. Such treatment may,

however, result in a liquid which is strongly corrosive and ero-

sive at the high ow speeds and pick-up speeds associated

with high-pressure pumping. The generation of high heads

per stage involves correspondingly high ow velocities in the

BFW system. The BFW system can be highly corrosive and

erosive at high velocities, since the protective lm of the salt

or oxide of the metal, normally found in static corrosion, is

eroded away by the high velocity of the system. This corro-

sion and erosion occurs despite the fact that there may be no

abrasive particles in BFW. The proper type of stainless steel or

alloy steel should be used in BFW pumps to prevent corrosion

and erosion.

The erosion-corrosion and the corrosion-fatigue have

been reported for BFW pumps. The erosion-corrosion is the

acceleration or the increase in rate of deterioration or attack

on a metal because of relative movements between a cor-

rosive uid and metal surfaces. Cavitation damage is usually

considered a special form of the erosion-corrosion, which is

caused by the formation and collapse of vapor bubbles in the

liquid near a metal surface. The corrosion-fatigue is dened

as the reduction of fatigue resistance due to the presence of

a corrosive medium. The corrosion-fatigue is also inuenced

by the corrosive to which the metal is exposed.

The oxygen content, temperature, pH, and solution com-

position can inuence corrosion-fatigue. The corrosion-fa-

tigue resistance might be improved by using proper coatings.

However, a coating is usually discouraged in BFW pump ap-

plications. A proper design with a correct material selection

is nearly always selected. Too often, the corrosion-fatigue

process could result in cracks in pump components. The

high-speed of BFW pump rotating parts favored the growth

of cracks, and nally the component could be broken. There

have been some unplanned shutdowns of steam generation

systems because of the corrosion-fatigue in BFW pumps.

The material selection is an important consideration for

BFW pumps. The use of suitable grades of stainless steel in

BFW pumps have resulted in better reliability, safety, and the

long lasting of various components and parts.

Reliability & AvailabilityHigh availability, usually above 99 percent, has been required

for a BFW pumping system. High availability is required in

BFW applications in order to keep the steam generation unit

running at its own maximum availability. Operators dont want

to have to shut down the plant (whether an industrial plant

or a power generation unit) for failure of an auxiliary pump

system. A standby BFW pump is necessary for nearly any

BFW pump system.

A considerable amount of attention is being given to suc-

tion piping and suction system performance. An important

reason is the possibility of cavitation. The size of steam gen-

eration units has been increased constantly in the last couple

of decades. The capacity of steam generation units has been

increased steadily in the last 50 years, which resulted in the

demand for higher capacity BFW pumps, higher speeds, and

more NPSHR. However, the height at which a deaerator is

installed (which is related to the NPSHA) has not been in-

creased with the same rate. In modern large steam genera-

tion unit designs, the height at which the deaerator is installed

and the BFW pump suction piping require special attention.

Pump ConfigurationMultistage BFW pumps are often designed and built in two

different configurations:

The In-Line conguration (also known as the Equidi-

rectional conguration).

The Back-to-Back conguration (also known as the

Opposite-Impeller conguration).

Advantages and disadvantages of the two designs are an-

alyzed and described in this section. In the selection, different

factors such as hydraulic, structural, dynamic, and operation-

al considerations should be respected. Particular attention is

required for the axial load balance and the lateral dynamic

behavior, with new and worn clearance conditions.

The in-line conguration is simpler, more compact, and in

many cases more efcient. In this conguration, the ow leav-

ing the impeller outlet is conveyed into the diffuser and then

to the eye of the subsequent impeller.

The back-to-back conguration consists of two groups of

impellers, with one group installed opposite the other group.

The number of impellers in the rst group is half of the total

number of impellers if the number is even. If the total number

of impellers is odd, the number of impellers in the rst group

of impellers is usually half of the total minus one. After the

rst group of impellers, the BFW ow is conveyed via two

crossover channels to the second group of impellers, which

are situated opposite the pump. During this crossing, the ow

is subjected to a pressure drop. The hydraulic efciency of

the pumpand as a consequence the overall efciencyis

affected by this pressure drop.

On the other hand, the in-line conguration brings a very

high axial load, due to the sum of the axial thrusts of every

impeller. A balancing drum is necessary to balance the thrust

and to reduce the load acting on the thrust bearing.

Back-to-back pumps are always well balanced (especially

when the number of stages is even), and the balancing drum

is less critical. This is an important issue, particularly when all

clearances begin to increase. Assuming a uniform wear of all

seals and rings, when clearances are increased with respect

to design ones, a back-to-back pump is still well balanced,

while for an in-line pump the axial load increases to a high

value, which could be 5-10 times the rated axial load of an

axial bearing.

Different ow leakages are reported on balancing drums

for both pump congurations. An in-line pump balancing drum


INSTALLATION GUIDELINES | Boiler Feedwater Pumps

is usually subjected to a total differential

pressure of all stages. On a back-to-back

pump, the total differential pressure

is usually subdivided in two balancing

drums. The diameter and clearances

of an in-line pump balancing drum are

greater because of a higher axial load.

Therefore, the balancing drum total leak-

age is greater for an in-line pump com-

pared to a back-to-back pump. Leakages

in an in-line pump drum could be 3065

percent more than ones in a back-to-

back pump. This could affect the volu-

metric efciency and the overall perfor-

mance of a BFW pump.

It is difcult to give a general in-

struction on which conguration is the

best for BFW services. Both congura-

tions are used today for different BFW

pumps. Generally, the best selection is

dependent on the application. However,

there is a preference for large, high-

pressure multistage BFW pumps.

The back-to-back conguration has

small hydraulic and technological dis-

advantages for the crossover channel

required to convey the ow from the

rst group to the second group of im-

pellers. On the other hand, it seems

more advantageous for the balancing of

axial load and for volumetric efciency,

mostly in worn clearance conditions.

Different dynamic studies indicated that

the back-to-back conguration can lead

to a rotor behavior, which is less sensi-

tive to the increase of clearances. The

damping factors of this conguration

are usually high. In most applications,

particularly large, high-pressure mul-

tistage BFW pumps, the back-to-back

conguration can increase the reliability

of the pumps. FC


Boiler Feedwater Pump | Cavitation | Corrosion | Power Plant | Pumping System | Steam

Find related content @ owcontrolnetwork.comSearch on:FLOWSTREAM

Amin Almasi is a senior rotating machine consultant in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng Mechanical), IMechE (CEng MIMechE), holds bachelors and masters degrees in mechanical engineering, and is a registered professional engi-neer in Queensland. He specializes in rotating machines, including cen-trifugal, screw, and reciprocating compressors, gas turbines, steam turbines, engines, pumps, subsea, offshore rotating machines, LNG units, condition monitoring, and reliability. Mr. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, subsea, and reliability. He can be reached at [email protected].


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S I N C E 1 9 5 8

Principle of OperationDespite their similarity, Coriolis and

ultrasonic flowmeter have very differ-

ent principles of operation. Coriolis

flowmeters are composed of one or

more vibrating tubesusually bent. The

fluid to be measured passes through

the vibrating tubes. The fluid acceler-

ates as it passes towards the point of

maximum vibration and decelerates as

it leaves this point. The result is a twist-

ing motion in the tubes. The degree of

twisting motion is directly proportional

to the fluids mass flow. Position detec-

tors sense the positions of the tubes.

While most Coriolis flowmeter tubes

are bent, some manufacturers have

also introduced straight-tube Coriolis

flowmeters.

There are two main types of ultra-

sonic owmeterstransit time and

Doppler. Transit-time ultrasonic owme-

ters have both a sender and a receiver.

They send an ultrasonic signal across a

pipe at an angle, and measure the time

it takes for the signal to travel from one

side of the pipe to the other. When the

ultrasonic signal travels with the ow,

it travels faster than when it travels

against the ow. The ultrasonic owme-

ter determines how long it takes for the

signal to cross the pipe in one direction,

and then determines how long it takes

the signal to cross the pipe in the re-

verse direction. The difference between

these times is proportional to owrate.

Transit-time ultrasonic owmeters are

mainly used for clean uids, while Dop-

pler meters are used for dirty uids.

Early Adoption IssuesUltrasonic flowmeters were introduced

by Tokyo Keiki in Japan in 1963, while

Coriolis flowmeters were first brought to

the commercial market in 1977. After

their introduction, both meters went

through a difficult acceptance period.

Many of the first ultrasonic flowmeters

were clamp-on meters, and end-users

had difficulty positioning them cor-

rectly. They also were not as accurate

as later inline ultrasonic meters. In

the 1980s, Panametrics (now part of

GE Measurement & Control, www.ge-

mcs.com) and Ultraflux (www.ultraflux.

net) did research on using ultrasonic

flowmeters for measuring gas flow.

By the 1990s, ultrasonic meters had

advanced technologically to the point

where they began receiving wider end-

FLOW UPDATE | Coriolis & Ultrasonic Flowmeters


Coriolis vs. Ultrasonic FlowmetersComparing and contrasting two popular solutions for flow measurement

It is interesting to compare Coriolis and ultrasonic flowme-ters, as a great deal of new product development is occur-ring with both of these meter types. In addition, Coriolis

and ultrasonic represent the two fastest growing flowmeter

categorieswith the possible exception of multiphase flow-

meters. Both Coriolis and ultrasonic flowmeters are widely

used in the oil & gas markets, and both are used for custo-

dy-transfer applications. The following article examines how

these two meter types are alike, as well as how they differ.

By Jesse Yoder, Ph.D.

0.2%

33.2%

53.3%

13.3%

Petroleum Liquids

Non-petroleum Liquids

Gas

Steam

2.1% 17.0%

39.5%

41.4%

Petroleum Liquids

Non-petroleum Liquids

Gas

Steam

Shipments of Coriolis Flowmeters Worldwide by Fluid Type in 2011

Shipments of Ultrasonic Flowmeters Worldwide by Fluid Type in 2011

Source: Flow Research, Inc.


user acceptance.

Early Coriolis owmeters had technical problems that

interfered with their acceptance. Problems with vibration

made it difcult to maintain zero point stability. End-users

found the large size and weight of even two-inch or four-inch

meters to be prohibitive. Their high price also presented an

issue for many end-users, as Coriolis owmeters are the

most expensive meter, even today. By the 1990s, some of

the technical issues with Coriolis owmeters had been re-

solved. In 1994, KROHNE (us.krohne.com) introduced the

rst commercially successful straight-tube Coriolis owme-

ter. This design addressed some problems with uid build-

up and pressure drop in bent-tube meters.

Differences In Line SizeCoriolis and ultrasonic flowmeters are dramatically different

in terms of line sizes. Over two-thirds of Coriolis meters are

made for line sizes of 2 or less. Until recently, the only

Coriolis flowmeter above 6 was made by Rheonik (now

part of GE Measurement). In the past five years, three more

companies have introduced Coriolis meters for line sizes

above 6. The companies include Micro Motion (a division

of Emerson Process Management, www.micromotion.com),

Endress+Hauser (us.endress.com), and KROHNE. These

meters are designed for line sizes of 8 to 16, and they are

mainly designed for custody transfer of oil and gas. While

their price tag can be as high as $75,000, higher oil prices,

and the increased value of natural and industrial gas, have

made it beneficial for some companies to pay for the higher

accuracy afforded by these large Coriolis meters.

While Coriolis meters excel in the lower line sizes, ultra-

sonic meters do best in line sizes of 4 and up. The larger

diameters make the differences in transit time of the ultra-

sonic signal easier to detect, although they can perform well

in smaller line sizes. Ultrasonic meters do not have the large

line size limit that Coriolis meters have, and it is common for

them to be made in sizes from 12 to 42, or even larger.

Insertion ultrasonic meters can be used in pipes of any size,

though so far no one has made an insertion Coriolis meter.

Likewise, clamp-on ultrasonic meters give ultrasonic tech-

nology more versatility in check metering and temporary

measurements. There is no clamp-on Coriolis owmeter.

Both Flowmeter Types Benefit from Industry ApprovalsCustody transfer of natural gas is a fast-growing market,

especially with the increased popularity of natural gas as an

energy source. Natural gas changes hands, or ownership,

at a number of points between the producer and the end-

user. These transfers are called custody-transfer points, and

they are tightly regulated by standards groups such as the

American Gas Association (AGA, www.aga.org). Other geo-

graphic regions have their own regulatory bodies.

One important function of the AGA and the American

Petroleum Institute (API, www.api.org) is to establish stan-

dards or criteria for sellers and buyers to follow when trans-


8OWUDVRQLF %XEEOHU S+ 6', 0RGEXV

FLOW UPDATE continued

ferring ownership of natural gas and petroleum liquids from

one party to another. In the past, these groups have pub-

lished reports on the use of orice-plate meters and turbine

meters for use in the custody transfer of natural gas. The

importance of these reports is illustrated by the example of

ultrasonic owmeters. In the mid-1990s, a European asso-

ciation of natural gas producers called Groupe Europeen de

Recherche GaziSres (GERG) issued a report laying out crite-

ria to govern the use of ultrasonic owmeters in the custody

transfer of natural gas. This resulted in a substantial boost

in the sales of ultrasonic owmeters for this purpose in Eu-

rope. In June 1998, the AGA issued AGA Report 9, which also

gave criteria for using ultrasonic owmeters in natural gas

custody-transfer situations. This caused a substantial boost

in the sales of these meters for that purpose, especially in

the U.S. The market for using ultrasonic meters to measure

natural gas for custody transfer is one of the fastest growing

segments of the owmeter market.

The AGA approved a report on the use of Coriolis owme-

ters for custody transfer of natural gas in 2003. This report

is called AGA-11, and it is, in part, responsible for the over-

all positive growth rate of Coriolis owmeters, which are now

widely used for natural gas custody-transfer applications. Even

though it often takes some time for end-users to adopt a new

technology, this report has provided a signicant boost to the

use of Coriolis owmeters for natural gas ow measurement.

The API has issued a draft standard entitled Measurement

of Single-Phase, Intermediate, and Finished Hydrocarbon Flu-

ids by Coriolis Meters. This document was added to the API

Library in July 2012. A second draft standard called Measure-

ment of Crude Oil by Coriolis Meters has also been approved

by the API.

Ultrasonic Meters Do Well On Both Liquid and GasBoth ultrasonic and Coriolis flowmeters do well on liquids.

Ultrasonic meters are widely used on both hydrocarbon

liquids and on water, and they perform well on both.

Multipath ultrasonic flowmeters, meaning those with three

or more paths, are used to measure the custody transfer

of hydrocarbon liquids. Coriolis flowmeters are widely used

to measure hydrocarbon liquids, especially for distribution

purposes downstream from a refinery. Many of these are

custody-transfer applications. Coriolis flowmeters are less

widely used in the water and wastewater industry because

Documents

Flow Control March 2014[1]