Centrifugal Pump Minimum Flow

Send a copy, someone here will certainly pick up the thread.

Look at the curve. This is a random curve.

If the pump curves show the NPSH_r at various conditions, the minimum flow rate will occur where the NPSH_r (plus maybe a safety factor such 2 feet) becomes greater than the NPSH_a. A basic curve may not be sufficient.

These are all good points--GA. The 50% of BEP is a familiar rule of thumb, especially if the pump curves do not include NPSH_r curves.

Pipes from the volute go to the lantern ring in the stuffing box - not to the bearings.

There are usually orifice plates in these lines and the flow would normally be too low to maintain thermal stability of the pump.

I thought more for "Mechanical Seal"!

Stuffing Box system must be adjusted so that there is a drop-leak to lubricate and cool the shaft. Usually that should be enough.

Mechanical seals need cooling in some cases and I tought this type of feeding back was for that Only....Please correct me.

Stuffing box will only leak if the suction pressure is greater than atmospheric, hence the need to supply cooling fluid at a higher pressure to the lantern rings in the stuffing box. This serves to cool, lubricate and prevent air ingress.

You are correct, the same holds true for mechanical seals if fitted. This would be an API plan 11 and would serve to maintain the seal chamber pressure above atmospheric pressure and to dissipate the heat generated by the seal faces.

Basha...

The only definitive source of information on minimum flow is from the pump vendor.

This is because there are mechanical issues ( shaft defelction...excessive bearing loadings) as well as heat build-up issues that also control the determination of a minimum flow

Generally speaking, single stage centrifugal pumps have a lower % minimum flow than multi-staged pumps. The pumps with the highest minimum flow requirements are multi-staged boiler feed pumps and the similar de-scaling pumps used in steel mills.

Since you probably do not have any documentation for this used pump, you will have to rely on a rule of thumb from someone.

The HI standard mentioned below is a good place to start.

Did you bother to GOOGLE your question before you came to this forum ?

What did you find out during your search ?

Minimum flow is zero. Assuming you shut off the outlet or have a very long vertical pipe, the pump develops a maximum pressure. The fluid in the casing just gets churned and heated. pump will not be damaged unlike a positive displacement pump. Bioramani

"pump will not be damaged unlike a positive displacement pump"

Sorry, but your information is incorrect. The centrifugal pump will definitely destroy itself. If you deadhead the pump, there's no place to dissipate the heat energy.

A centrifugal pump has a continuous passage from inlet to out let. The velocity head is converted into pressure head.

Unless the inlet has a one way valve (The OP did not give any layout) the pump will not be damaged.

Bioramani

I don't know where your head is, but it must be really dark there. You are still wrong.

If inlet and outlet are blocked there is no flow, now way for heat to escape. Pump fails. End of story!!

With all respect to you, I did say 'unless there is a check valve'. The curve that you have provided above has a value of zero flow at dead head pressure. A flooded suction centrifugal pump with no check valve takes quite a while to heat up once enough vapour is produced the pressure developed drops quite a bit. It does not blow up like a positive displacement pump pumping an incompressible fluid. bioramani

Sorry, the curve was generic. I'm not talking about explosions, I'm talking about pumps building sufficient internal heat to destroy themselves.

This discussion (OP) has nothing to do with PD pumps. You first introduced the subject of PD pumps in your post #9.

Up to that point, the discussion had been confined, properly, to centrifugal pumps.

Sorry If I offended you.

usually those pump curves ends before "0" flow.

Zero flow is one of the required points when testing API 610 pumps and is a required point on the curve. One reason for this is to see whether the curve rises continuously to Shut off as required.

Generic families of curves obviously do not show this point.

As you point out, the short term problems of running at shutoff are related to catastrophic failure of seals due to dry running. As vapourisation of the pumped medium takes place, the point of highest temperature is at the seal (or packing) faces. If the seal then fails, the vapour that has formed in the pump due to the increase in temperature is very often a flammable one. Loss of containment is a very serious issue in many plants.

The long term issues are more to do with damage to seals and bearings due to high axial and radial loads due to non-ideal hydraulic behaviour within the pump at low or no flow. This leads to a reduction of the time between maintenance.

Running a centrifugal pump at very low flow or shutoff head is without question, something to be avoided.

Thats is true, we at deadhead centrifugal pumps at times with a modulating valve to control flow.

Ditto Lyn, only idiots will dead head a pump continuously. In fact some "operators" think they can save enrgy by not switching off the pump and then ONLY shut off the valve, avoiding frequent on/off. However, as in my tenure as engineer at a yeast plant, 24hrs deadhead makes a Sulzer volute go bang.

The fluid in the casing just gets churned and heated. pump will not be damaged. This is the sad part of this forum, or any other forum for that matter. The OP started his post "dear engineers", and then somebody writes this drivel. He is obviously not an engineer.

The liquid will heat up against a closed valve, and if left to churn, it will explode due to excessive pressure. Anybody that have seen a blown volute will know. Even if it does not explode ( leaking seals, maybe) it will destroy itself.

Under most circumstances, this is incorrect, although it can be true in unusual circumstances.

The worst problem occurs if liquid is trapped in the pump by closed upstream and downstream valves (or check valves). This rare scenario could heat up the trapped liquid, leading to hydrostatic expansion and possible rupture after a period of time. If the liquid can flow back into the suction, or through a relief valve, no such excessive pressure will occur.

The churning of static fluid will indeed heat it, as Bioramani mentions. This might cause a seal to leak, or even boil the liquid back through the pump suction. But again, if the liquid is not blocked in by closed valves in both directions, there will be no harmful rise in pressure.

This differs from positive-displacement (gear, vane, piston, screw) pumps, in which closure of a discharge valve will cause abrupt pressure rise.

Your statement is much too alarmist and dogmatic, and not particularly knowledgeable.

of course not, only gurus seem to be.

gsbasha -- SimS gave you a good generic answer in #4. If you want some specific info tell us more about the application you have in mind since running a centrifugal pump at low flow can have substantially different consequences depending on the size, specific speed and impeller diameter relative to available maximum impeller diameter of the pump as well as the system head curve of the piping with respect to the proportion of static head to friction head. Also if you have a specific pump on a specific system in mind tell us as much as you can about it.

Generally the larger the pump the bigger will be the issues of inefficiency and bearing loads from operating near to shutoff. In small fractional HP pumps these issues become trivial compared to the initial procurement cost of the pump. And note that it doesn't take much actual flow to carry away the waste heat absorbed by the water in the casing of the pump.

Another thing to be aware of is operating a pump with rising head -capacity curve at low flows on a system with mostly static head can invite surging as the pump hunts between two close possible operating points.

Ed Weldon

As alluded to in earlier posts- there is DEFINITELY a minimum flow.

If you do not have the pump curve, assume an efficiency of 50% (likely at low flow) - meaning that 50% of input energy has to make heat. The minimum flow would be whatever is required to keep whatever flow is occurring from being heated more than about 10F.

Example- a 20 HP motor at 50% efficiency generates 50% X 2545 BTUH per HP X 20 HP, or 25,450 BTUH. That thermal input, divided by 10F rise is 25450 / (8.33 pounds per gallon X 60 minutes per hour) or 5.1 GPM. That is the absolute MINIMUM flow. To be safe, I would double that to ease the burden on the likely metal-ceramic seals, so consider minimum flow for a 20 HP motor pump to be about 10 GPM.

Dear all,

Can someone confirm the folllowing for me: Is Static Head only applicable in Open systems? For example in a closed system (tank, pump and recirclation loop) does the static head actually matter since the liquid on the pump discharge side is being "pushed" by the liquid on the suction side. If this is correct then is the head only a result of the pipework friction (which is a result of the flowrate). Thank you in advance for your input.

John

Yes. Static suction head and static discharge head will be the same.

NOT TRUE!!

Consider your cardiovascular system, which is a closed system. If your systolic pressure is equal to your diastolic pressure you have been dead for some time and your heart is not beating.

Where . . . have all the engineers gone . . . ?

They are still arround these engineer!

But your comparison does not stick as an example: Systolic diastolic pressure are not Static pressures! The previous remark was about static pressures only (I think).

Therefore, find another example and try to be more accurate so that we could follow you. Thank you.

fjdomingues,

"Where . . . have all the engineers gone . . . ?" how ironic given your answer. Are you actually an engineer? I somehow doubt it. I am guessing that you are either a hairdresser or work as a manager in McDonalds.

Oh dear!

By my way of thinking a closed system has a static head - the pressure existing in the system without the pump running.

An open system has by definition a static head proportional to the height of the liquid in the expansion tank.

Static head is the height which a pump can pump to. It is actuall y the pressure exerted on the system prior to delivery.

Your two statements are mutually contradictory.

Jvrj: YES! You are absolutely right. Static head – after the pump is turned on – is the height which a pump can pump to.

Energygod introduced the work 'dynamic'. That as well refers to after the pump turns on, and more correctly – DYNAMIC STATIC HEAD.

I think Guest in #21 was ruminating about the pressure that exists in a system before the pump is turned on - it is the true static head ON THE SYSTEM. In an open system it is the height of the water above the pump, in a closed system it is the pressure exerted on the diaphragm of the bladder expansion tank or the air in a conventional expansion tank.

The problem is when THIS static head is insufficient when the pump starts. If the suction pressure on the suction side of the pump is pulled down below the vapor pressure of the fluid being pumped we get cavitation.

After the pump starts we call the suction pressure on the suction side of the pump Net Positive Suction Head (NPSH), because it had better positive (I guess) or the fluid boils – cavitation.

One must analyze the phrase "Suction Pressure". Even though there is suction, there still is pressure. It might be negative, but it is still pressure.

In our bodies the difference between systolic and diastolic pressure it the true DYNAMIC STATIC HEAD pressure produced by our hearts. The true static pressure of or blood – a closed system – is greater than atmospheric pressure, and that explains why the blood leaks out when we cut ourselves.

Fortunately blood has a higher vapor pressure than what the suction pressure gets pulled down to when our heart beats. If it didn't our blood would boil.

My blood often boils, but due another reason.

Must get back to work, but this is more fun.

Static head is when nothing is moving; it is just the difference in elevation between where the pump is pumping from and where it is pumping to. After the pump turns on, static head no longer applies alone; it is now dynamic head (things are moving). At this point you need also to consider pipe friction losses, and also velocity head at the smallest cross-sectional area in the piping system. The term for all of these is "total dynamic head", or TDH.

Total dynamic head plus static head = Total head.

Delivery was brought into my statement MEANING the point at which the pressure escapes ( or ends its journey under pressure. The end of the pipe, out of the pipe, now in a tank or dam. ie. the pump has done its job, whether its 3 meters or 30 meters.

"Static head is the height which a pump can pump to"

I guess this is not what you meant to say. Static head is a system head - it has nothing to do with the pump.

Thank you,

The only "watch-out" is a possibility that the combined static and dynamic pressures might exceed pump volute or seal failure pressure. If that happens, you hear a POP and have a big mess plus non-functioning system to deal with until you buy and install a new, stronger pump.

the pump should give (mod/ser plate) the gpm's and rpm's, i think at minimum flows, the siphon may break, and cavate on the suction side of the pump.

post a copy of the data sheet for all to see please