Sealing Problems

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Amongst the technologies

developed for oil and gasproduction in the last 20

years, multiphase pumping is one of

the most promising. The significant

reduction of capital investment either

on or offshore together with an

increased production rate has led to a

growing demand for pumping systems

to be installed.

The biggest challenge to be overcome

by a piece of rotating equipment in

multiphase service is the variation inthe process fluid from 100% liquid to

100% gas. This creates very difficult

operating conditions for the pump andeven more difficult conditions for its

mechanical seals, which are a critical

part of any pump.

Mechanical seals are readily available

for liquid or gas phase applications.

However, in a multi-phase pump the

mechanical seals will see

transient conditions. The transient

nature of the wellhead stream creates

pressure surges, slugs of liquid and gas

locks. A multiphase pump and itsmechanical seals must be able to

withstand all these rather harsh oper-

ating conditions. Consequently specialmechanical seals have to be developed.

Types of pump

Multiphase pumps can be subdivided

into two groups, each with benefits

and limitations. These are

rotordynamic and positive

displacement pumps.

Rotordynamic pumps rely on theconcept of helico-axial hydraulics. A

Mechanical sealing technologyused in multiphase pumping the benefits

Nikolaus Necker from Burgmann Industries GmbH & CoKG describes theadvantages you can gain when using mechanical sealing technology inmultiphase pumping.

f e a t u r e m u l t i p h a s e p u m p s

WORLDPUMPS August 2005 www.worldpumps.com

Figure 1. Pressurized

dual mechanical seal(API 610, plan 53 or

54), type SH, for

multiphase

pumping.


2/3WORLD PUMPS August 2005 www.worldpumps.com 21

multistage pump can have up to

15 compression cells on a

single shaft. Each compression

cell comprises a rotating helico-

axial flow impeller and a stationary

diffuser.

The produced effluent can thus

be boosted over long distances

without the need for prior separat-

ion and with no limitation regard-

ing gas volume fractions. This type

of pump is compact, offers mech-

anical simplicity and reliability,

is tolerant of solids and produces high

flow rates.

Positive displacement pumps, or

twin screw pumps as they are alsoknown can already handle gas

volume fractions up to 70%. In this

respect only some minor improve-

ments have to be made to existing

pump design. The twin rotor screw

pump consists of two contra-

rotating screws.

A certain amount of liquid is

needed to seal the gaps between

the screws and the liner. This kind

of pump has true dry runningcharacteristics, thus can handle

an extreme range of specific

gravity and gas volume fraction.

They have proven their re-

liability in several real multiphase

boosting applications.

The problem task

Unlike sealing systems, which are

exclusively and specifically applied

in liquid or gaseous media,

mech-anical seals for multiphase

pumps operate with continually

chaning gas and liquid flow rates.

Thus the mechanical seal has

to cope with the following

conditions:

unpredictable process medium

composition

density and viscosity variations

temperature variations

erosion effects, mainly by sand high and low operating pressures

insufficient lubrication and

cooling of the seal faces.

Chemicalparameters

The chemical analysis of the process

medium differs, depending on the oil

field and/or the location. This means

that the crude oil may contain:

wax

salt

water

sour gas (H2S) and / or carbon

dioxide (CO2)

cracked hydrocarbons

solids.

All this affects the operating

performance of the seal and results in

erosion or corrosion of the mechanicalseal components.

Sealing principle

The fluid being sealed decreases in

pressure across the width of the seal

face. The friction at the seal face

results in a temperature increase in the

fluid at the seal face. The decrease in

pressure and the simultaneous

temperature increase of the fluid at the

seal face results in the vaporisation of

the volatile elements within the

sealed fluid.


Figure 2. Rotordynamic Pumps rely on the

concept of helico-axial hydraulics.

Figure 3. The Twin Rotor Screw Pump consists of two contra-rotating screws.

Figure 4. A typical

dual high pressure

seal in a face-to-face arrangement.


3/3www.worldpumps.com WORLDPUMPS August 200522

This can result in insufficient

lubrication and cooling of the seal

faces. The gas content in a multi phase

process medium creates an additional

risk of insufficient seal face

lubrication which may result in the

failure of the seal faces.

In an optimum parallel gap the axial

opening and closing forces are

balanced. In this case the contact

force (Fc) is evenly distributed across

the width of the seal face. The

pressure gradient of the sealed fluid

(Fs) is linear.

Depending on the mechanical

seal face stiffness, two other gap

formations can occur.

In an A-gap situation a high contact

force at the outside diameter is

created. The pressure gradient

steepens. This results in an unstable

running condition of the seal.

In a V-gap situation, a low contact

force is created at the inner diameter

of the seal face, the pressure gradient

shallows, the mechanical seal runs in

a stable condition.

Two examples of rotating sealing

solutions follow.

Single mechanical seal(API 610, plan 32)

Single mechanical seals can be used

in twin screw pumps for multiphase

applications. Single seals are

lubricated and cooled by a flush from

an external source (API610-plan

32). The flush has to be continuous

and reliable even during start-up

and shut-down of the pump. A close

clearance throttle bushing in front of

the seal controls the rate of flow of

the flush medium into the pump,

thus isolating the seal from the

process fluid.

Pressurised dualmechanical seal (API610, plan 53 or 54)

Engineered pressurised dual seals

are predominantly used in multi-

phase pumps. The example shows

a typical dual high pressure seal

in a face-to-face arrangement.

This type of seal is commonly

used in areas where the practicality

of using a single seal with

plan 32 external (continuous loss)flush is limited. Dual seals

are lubricated by an external closed

loop supply system.

This mechanical seal is a special

design employed for high duty sealing

applications. The seal faces are

double balanced; in the event of

barrier fluid pressure loss or a high

pressure transient in the seal chamber

the seal remains closed allowing

pump rundown with little risk of face

damage or product contamination to

the atmospheric side. The identical

stationary seal face assemblies give

optimum running characteristics

under changing operating conditions.

The seal is designed as a cartridge

unit, thus is compact, robust and easy

to maintain.

Corrosion and erosion is avoided by

choosing appropriate seal face andconstructional materials. Silicon

carbide is typically used for the seal

faces as it has high hardness and

excellent heat transfer properties.

Duplex and Super duplex stainless

steels are used for the metal parts.

Conclusion

Mechanical seals have proven

their ability to seal multiphasemixtures over a wide range of

operating conditions in hundreds of

applications in extremely harsh

environments around the world.

The mechanical seal types and sliding

materials available today are capable

of meeting any technical demand

defined by multiphase pumping

services. In order to assimilate the

sealing systems with the specific

operating conditions close co-

operation between seal manufacturer,

pump manufacturer and end

user is a basic precondition. Correct

application, installation and

operation are the key factors for a

long meantime-between failure. s

CONTACTNikolaus Necker

Burgmann Industries GmbH & Co. KG

Aeussere Sauerlacher Str. 6-10

82515 Wolfratshausen

Germany

Phone +49 8171 23 1200Fax: +49 8171 23 1214

E-mail:[email protected]

Website: www.burgmann.com


parallel-

gap

A-gap

V-gap

gap and contact

force

indifferentFS

FC

pa > pi

k1 = 0,5

0 < k1< 0,5

0,5 < k1

< 1

unstable

stable

Figure 5. Mechanical steel gap stiffness.
mailto:[email protected]:[email protected]:[email protected]

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Sealing Problems