Selecting Pumps

8/8/2019 Selecting Pumps

http://slidepdf.com/reader/full/selecting-pumps 1/4

the chemical engineer

www.tcetoday.com

issue 808, october 2008

food breaking with tradition

assets managing money and maintenance

graduates chasing the world's most wanted

ChemEng08



MAKING sure you have the right

pump in the right place and that it’s

working correctly appears to be a

simple exercise. However, often the job

turns out to be more complicated than

you expected. This article points out a

few key aspects of pump selection and

evaluation to help you pick the right

equipment and get the most out of it.

pump selection

Selecting the correct pump for a

particular service can at first seem to

be a straightforward task. Mechanical

engineers tend to start with a process

data sheet where process engineering

has advised how much of a particular

liquid has to be moved and where

it needs to go. This gives fluid

characteristics, a flowrate and pressure

– the basic parameters for selecting a

pump.

Nowadays, computers tend to be used

for pump system calculations using the

suction and discharge vessel pressuresalong with the process parameters and

piping detail to calculate the system

losses for the given liquid flow. Some

software even suggests a suitable pump

type but I would rather leave that to an

experienced equipment engineer!

The material to be pumped is of

course determined by the process

and the reactions in the process. The

physical properties of the fluids are

known. That is, viscosity, density and

vapour pressure, which are all dependent

on temperature.

While the above description may

seem simple – find a pump to transport

a set quantity of a liquid to some other

place – this is deceptive. Chemical,

petrochemical and refining processes do

not run on constant conditions. Nothing

is that simple. Flowrate changes, as

does pressure. Temperature is likely to

vary at different times throughout the

process and this in turn affects viscosity,

density, vapour pressure and so on. This

is likely to result in pressure variations

in the system.

All of these variables very much affect

the type of pump that can be used and

indeed, how it is used, so selecting thecorrect pump is not always that simple.

If the mechanical equipment engineer

does not know the whole story, the

stage is set for problems. An equipment

engineer who suspects he does not have

the whole story of how his equipment

will be used and what might be

expected from it would be well advised

to carefully check and question the

given details, and perhaps ask leading

questions!

varying flowrates

Most pumps are required to pump atvarying flowrates; therefore both the

maximum and minimum requirements

need to be specified. All pumps have

limitations. For example, a centrifugal

pump operating with a flowrate that’s

either too low or too high could well

face problems with axial or radial

thrust, net positive suction head (NPSH)

requirements exceeding that available,

vibration and so on. Over-estimating

the flowrate can also result in problems

during operation. An equipment

engineer will tell you that over-

estimating the flowrate by 10% will only

land you with a piece of equipment that

is too big and not working at its most

efficient or ’healthy’ operating point.

It often pays to keep a pump operating

pumps

www.tcetoday.com october 2008 tce 41

at its optimum load, so in the case of

the above-mentioned centrifugal pump

struggling with variable flowrates, it

could well be viable to fit it with a

variable speed drive. This would allow

for automatically adjusting the pump to

match the requirements of the system.

Obviously this doesn’t come free but the

energy saving made by always running

the pump at optimum often makes up

for the capital cost.Particular attention must be paid

to pumps operating in parallel. If

one parallel pump fails or shuts

down, the others could ’run out’ on

their performance curve to a point of

cavitation or where

they demand more

power than the driver

can supply.

Similarly, over-

specifying differential

pressure at the design

stage can lead to

similar problemswhen the pump is run

in the true installed

conditions (ie the real

system curve).

temperaturevariations

Pumping temperature

is obviously

important. The

mechanical

properties of pump

material change

with temperature,

especially extremes

of temperature.

Liquefied gases that

’auto refrigerate’ on

The G4T pump is used to inject CO2

into the North Sea. Photo courtesy

of Lewa

Below: Very large vertical

seawater intake pump.

Photo courtesy of Ebara

tce is published

monthly by the

Institution of

Chemical Engineers

Editorial: claudia@

icheme.org

Subscriptions:

[email protected]

Advertising:

nigel.stephens@

mainlinemedia.co.uk

www.tcetoday.com

Process engineers should keep in close contact with equipment engineering

colleagues, especially when selecting pumps, Peter Stevens explains

The right tools for the job


http://slidepdf.com/reader/full/selecting-pumps 3/442 tce october 2008 www.tcetoday.com

speed one and thus more expensive.

If possible, NPSH available should be

increased by elevating a suction vessel

or measures such as increasing the

suction pressure slightly, reviewing

suction pipe layout/sizing, or reducing

temperature and thus vapour pressure.

If any of these options is practical, the

resulting saving could be significant.

varying viscosity

Varying viscosity will result in varying

pump performance in most pump types,

most significantly with centrifugal

machines. A high-viscosity liquid needs

a much larger motor for a cold start-up

than for normal operating conditions. It

may be possible to warm up the system

at a low flowrate to reduce the viscosity

and thereby reduce power consumption.

When the viscosity is reduced the pump

can be operated at full design flow.

However, if high viscosity can occur at

any time then the pump needs to be

selected for this and motors must be

sized accordingly.

density dilemma

The absorbed power of a centrifugal

pump is directly proportional to the

density of the fluid. Pump drivers

must be sized for the worst operating

case, which may be highest density.

But again it may well be possible that

this higher density is only a temporarycase, often for start-up, and the pump

can temporarily operate at reduced

flow, cutting the power consumption.

A smaller motor could be installed

together with smaller cable, starter and

so on. Careful consideration of density

at various operating conditions could

mean the difference between installing

a low-voltage and a medium- or high-

voltage motor.

don’t supersize

A marginally-oversized process estimate

could make all the difference betweenan ordinary driver and a larger one

requiring force feed oil lubrication. I

have had cases where a quick check of

the process parameters has made all

the difference between an oversized

energy-hungry driver and standard

one with normal lubrication. This is

a very considerable cost and space

saving particularly if there is more

than one pump set involved. This also

brings to mind an old saying: “If you

do not require a piece of equipment,

you have saved all that money in

capital cost, installation, operation

and maintenance. But best of all if you

do not own something, it cannot go

wrong!”

In another instance, a site expansion

release of pressure

may require

pumps made out

of specialist or

unusual materials.

Such possibilities

need to be

highlighted.

Pumps are

sometimes

required to

withstand

’steam

out’

conditions,

perhaps to clean

out or decontaminate

the pump for inspection

or repair. This is often a higher

temperature than normal operation and

not only affects the pump itself butalso such items as gaskets and ’O’ rings

in the mechanical seals. ’Steam out’ is

not always noted on initial data sheets

but can have significant impact.

Some process reactions or ‘upset’

conditions can result in high

temperatures or higher than normal

suction pressures. The pump engineer

needs to be aware of this when

specifying a pump. Such conditions

may only occur when the pump is

static; in other applications the pump

may have to operate under these

abnormal conditions. This may well

require a different design, sealing

system, material selection or other

considerations.

Temperature has of course an effect

on vapour pressure and thus the NPSH

available. Suction and NPSH conditions

need to be calculated and considered

for all operating cases. Make an error

here and you’ll create a significant

problem or even catastrophic pump

damage. Perhaps the one parameter

that affects the pump selection more

than others is NPSH. If at all possible,the system should be designed to give

the best possible suction conditions

for the pump and not simply to prevent

cavitation. A higher NPSH available

could well mean the difference between

being able to select a simple single-

stage horizontal end suction pump and

having to use a vertical multistage

canister pump installed in a concrete

pit in the ground – two very different

pump types with two very different

price tags (not to mention installation

and maintenance) to achieve the same

basic duty. Another alternative may

be a slower running double suction

(impeller) pump for a lower NPSH

requirement. However, slower pumps

are bigger than a comparative higher

project called for an additional

centrifugal pump. A simple review of the

existing equipment revealed that the

existing pump had more than enough

spare capacity for the new requirements

– without any modification whatsoever.

All that was needed was a slight change

to the control valve. No new pump, no

foundations, no excavation, no added

power cables or motor controls, no

pipework, valves and so on. And the

pump would be operating at a more

efficient duty point. One less pump to

service too!

it’s good to talk

What is absolutely essential is that

the process engineer and equipment

engineer appreciate each another’s

roles and constantly communicate

with each other. This is especially

important if there are changes in process

requirements in the physical properties

of the pumpage.

Another source of potentially very

useful information is of course the

operators on the plant itself. It is quite

surprising what one can find out from

such sources. It is unusual, but does

happen that pumps are completely

wrecked once in operation. In one such

case, we eventually discovered that the

operators had a habit of ‘throttling’ the

flow by means of the suction valve! Not

something that the pump supplier ordesign engineer would have thought

about without investigating.

whose fault is it?

When a pump fails it is usually the

pump’s fault. More to the point, it’s

the pump that gets the blame. In fact,

most of the time it is the selection,

installation or operation that determines

how well a pump will work, or indeed

fail. After the duty requirements are

known, the equipment engineer uses the

process data to produce the mechanical

data sheet, which in turn is sent with

some form of requisition to a number

of suppliers. The suppliers select and

propose a suitable pump. Then, the

evaluation begins.

commercial evaluation

Commercial evaluation does not

generally impact mechanical engineers.

However, there are of course some

occasions when it does become

interesting. The capital cost of a piece

of machinery is one consideration, but

then there is also the running cost.

A few percentage points difference

in efficiency of a high energy pump

can mean a great deal of difference

in operating cost. As a very simple

example, 4% difference in efficiency of a

Above: High-speed

pump. Photo

courtesy of Sundyne

pumps



5 MW driver at a cost of €40/MWh could

be worth €320,000 over five years.

An old generalisation was that a one

percentage point saved in efficiency

would pay for the cost of a (bare-shaft)

pump during its operating lifetime.

technical evaluationTechnical evaluation on the other

hand is of much greater interest to

the equipment engineer. It very much

seems to be their mindset to take

things to pieces and see how they

look and work in detail. Essentially

that is what is done in a technical

evaluation but on paper, or rather

a computer spread-sheet nowadays.

The actual requirements are compared

with the pumps proposed by the pump

suppliers in their bids; basic design,

orientation, construction, impeller type,efficiency, absorbed power, suction

capability, metallurgy, compliance

with specifications, scope of supply,

quantities, testing and so on. Typically,

we also review reference installations

of similar machines on similar service.

With larger pumps, higher pressures

and more exotic requirements, it would

be naïve to purchase a machine from a

manufacturer without a successful track

record. For example, this is particularly

relevant with respect to thick section

castings in exotic materials.

Another area that has over the

years become more and more important

is pump noise. In many cases there

are maximum permissible noise

levels; sometimes this is enshrined in

legislation, and plants may be asked

to reduce noise levels as part of their

environmental consideration. Another

aspect is the reduction or complete

exclusion of emissions of hazardous

or toxic materials from equipment.

Mechanical seals and sealing systems

can only do so much though this

technology continues to develop,

particularly in the area of dry gas seals

(now extensively used for process

gas compressors). However, glandless

technology is making inroads, in thepump market in particular. The size

and capabilities of such machines

continues to develop in leaps and

bounds, particularly in canned motor

and magnetic drive pumps.

A thorough evaluation of vendor

bids should include a review by other

engineering disciplines including

electrical, process, piping, metallurgy

and insulation. Thorough evaluation is

the watchword here: it is far better to

spend the time to check and double

check all parameters at this stage,

before the machine arrives on site

with, for example, an undersized motor

or connection flanges pointing in the

wrong directions for the piping.

summary

Process conditions and the increasing

complexity and scale of plants

continue to challenge suppliers and

the capability of the equipment they

have to offer. Pressures are rising,

temperature ranges getting more

extreme, and machines are getting

larger requiring more powerful drivers,

bigger castings and forgings in exotic

materials. All of these requirements

have to be satisfied, along with higher

plant production rates, more stringent

environmental requirements, higher

availability (mean time between

failure), lower running and maintenance

costs, and if possible lower capital cost.

I am sure that for the imaginable future

there will be a great deal of work for

the pump, the people that apply them,

and the people that engineer them,

manufacture them and use them.

After all it’s only a pump! tce

Peter Stevens

(peter_stevens@

fwuk.fwc.com) is

principal rotating

equipment

engineer (Foster

Wheeler Energy)

pumps

www.tcetoday.com october 2008 tce 43

Documents

Selecting Pumps