Monday, January 16, 2012

If NPSHa is lower than NPSHr

If NPSHa is lower than NPSHr, What Changes can be made on Pump only, without changing flow rate?

Posted by Amir Hamdan, UAE at Pumps & Systems Group of LinkedIn

Responses –

(1) Venkitaraman Govindarajan, Singapore • Hi, in my view it's like cutting the feet to suit / match the shoes. NPSH r is the requirement of the pump. We need to ensure that the system is modified to meet this requirement. Hence, please focus on the system rather than the pump.

(2) Carlos Troyo, Costa Rica • Not much you can do on the pump without changing flow. Use the propper pump. Or instal the pump in a lower location were more head is available, and revise pressure drop on the suction pipping (again system design).

(3) Randal Ferman, LA, USA • It depends upon the specific pumping conditions and particulars of the pump inlet and impeller eye geometry. If the NPSH Margin Ratio (R) of NPSHa / NPSHr is not much less than 1.0 then it’s likely that some effective modification of the pump inlet or the impeller eye can be done. Pump manufacturers routinely make such modifications during testing to meet performance guarantees.

(4) Asif kundi, Pakistan • NPSHa must be greater than NPSHr. Going for changing geomatery and adding modification its better to reselect the pump type. And only that pump in the manufacturer range must be selected which NPSHr is lower or suits your requirement.

(5) George Nicolaidis, Greece • if you can raise the tank you're pumping from it may do the trick, or as said further up if you can lower the pump. If you're not prepared to lose capacity not much you can do... Just a thought that occured to me just now! How about pressurising the tank you're pumping from! That may do it!

(6) Peter Berghs, Calgary, Canada • See if the pump manufacturer offers an inducer that will reduce NPSHr. What fluid are you pumping? It may be possible to suppress cavitation by playing with partial pressures and injecting air or another gas into the inlet (<1%). As always in these discussions we need more info to provide accurate answers.
Or...Put a low head, low NPSHr booster pump in front of the unit you already have...this depends on if a low enough NPSHr booster pump can be found that meets your process conditions.

(7) Amer Hamdan, (He started this) • Fluid is Caustic Soda ,other pump is for EDC,

(8) Peter BerghsI had an amine booster pump that cavitated...adding a very small bleed of natural gas into the suction (needle valve cracked ! turn) just before the impeller significantly reduce the amount of cavitation damage. The effect of the gas could be heard. Check with your process staff to see if you can bleed in some nitrogen or other stable gas into your fluid....this will suppress the formation of pumped fluid vapor bubbles...I am not very familiar with caustic fluids so I can't say for sure.

(9) saurabh sitesh, Qatar • what can be done is to increase the NPSH availability..this can be done by using a vertical can type pump( VS6).. the length of the can is designed such that the difference in NPSHa and NPSHr is minimum 0.5m.

(10) Hans Fontijne, Netherlands • Its difficult to give you a answer thats the solution of your problem
normaly you have to check the situation

what is the medium / material you have to pump
temperature / viscosity?

high viscoity and low temperature can gives a low NPSHa

the length and diameter, how many elbow, valves, strainers are fitted in the suction pipe, al that gives more frictionloss.
Old pipe and others can also give more frictionloss

what give the suction gauge and pressure gauge,
the suction gauge can say the suction pipe is or get smaller (blocked). you have to clean the suction pipe inside

check first what is the real flow and what is the flow in the BEP (best efficiency point), is the fow more to the right
you need a bigger pump whit a NPSHr

if the flow more left of the BEP and the NPSHa is lower than NPSHr you have to redesign the diameter of the suction pipe (bigger diameter) or place the pump on a lower level, or use a other type of pump a vertical submersible.

Do not forget suction or persure cavitation can damaged your pump, think broken shaft, baerings, leaking seals.

(11) sunil gupta, Mumbai • if your NPSHr is lower than the NPSHa than first upon you have to analyse the route cause of the available NPSHa,it might be due to poor suction condition that are to be incorporate due to Various losses occur at suction pipe line.so due to which cavitation occurs and your required flow will be affect.Also it depends on type of media you are using for pumping.
And if your suction condition is poor than you have to design the new valve & valve seat assembly such that the NPHSr of your pump will be lower.so that it would not affect your flow.

(12) Simon Bradshaw, ITT, NY • Probably the best fix is to fit a low NPSHr impeller, specifically designed for the operating conditions.

If the NPSHr is not much below the NPSHa and you are not pumping an aggressive fluid then profiling and cutback of the impeller leading edges may yield sufficient improvement. However there are negatives to doing this.

If the inlet design is poor then it may be possible to rework it to reduce the losses and provide more uniform flow into the impeller eye. However most reputable manufacturers will have already optimized the inlet designs. CFD is your friend in evaluating if worthwhile inlet improvements are possible.

(13) Yasser Abdalla Grundfos Egypt A bigger impeller that can be installed in the same casing will have lower NPSHr. This could be available if you consult your pump supplier.

(14) Anis Hamdani Zuberi, Pakistan • You can do the either of the following things:

1. Insert a smaller diameter Impeller in the pump (if the duty point is towards the right of best efficiency point with the existing impeller diameter)

2. The Pump foundation should be lowered (below the suction tank) to increase the NPSHa.

3. If hydraulics of the pump permit then install a lower speed pump (it will have lower NPSHR than the existing high speed pump)

4. Reduce the number of bends, increase the Suction Pipe Diameter to increase NPSHA

(14) John Yatcilla, Netschz, Philadelphia, USA • If you don't want to lose flow by slowing the speed of the pump down, then look into making the suction piping size larger to help with the NPSHa.

(15) Bo Dixon, Colorado, USA • Have you thought about adding a "booster/charge" pump into the equation?

(16) Todd Vencill, Wyoming, USA • Randal Ferman is correct, you can modify the A and B gap some of the eye of the impeller to improve NPSHr and EFF in some cases However, not much. Depends on the style of pump and impeller. I would really look at the system first. If an Axial split case Multi-stage API-610 you can do some of this for sure. Most manufacturers will do this as Randal states to meet hydraulic tests and increase EFF a point or two.

(17) Amer Hamdan (He started this) • For your information, the design or elevations of tank ,suction line, elevation of pump cannot be changed due to project requirements, even a booster pump is not acceptable, the fluids are EDC and Caustic Soda, the tanks are open to atmospheric pressure, Now I have two questions: 1- Is the Inducer is a common used solution without any defect on duty point or pumps components? 2- both Pumps are API 610 ,is the difference between NPSHr and NPSHa accepted to be .6 M? Kindly advise.

(18) Randal FermanAmer, I believe just about every pertinent option has been offered by knowledgable pump experts in the foregoing comments. An inducer is just one of a variety of possible solutions. Concerning NPSH margin, even 0.0 m could be acceptable if conditions are known with a high degree of certainty. In my opinion you are at the point where you need to have an experienced pump professional directly involved.

(19) Amer HamdanThanks for your advise Mr. Randal, however OEM of these pump solutions were not acceptable after Technical evaluation of End user, As follows:
To use smaller capacity pumps with low NPSHr, as per the hydraulics of these pumps
: not in all cases the smaller pump has lower NPSHr ,it is related to design of casing and impeller, ( we are talking about ship loading pumps) and this will not be ok due to loading time required for each ship in port as it is time limited for loading ship, and the storage tanks in and out flow rate is calculated to keep certain minimum liquid level, so small capacity pump is not valid solution, and minimum liquid level cannot be changed

To add an orifice plate on discharge line of loading pump will lower the NPSHa but it will decrease flow rate by 20% and this is not ok, due to loading time limitations of each ship.

To lower Pump n: Speed is not valid as well due to loading time.

Finally OEM rejected to design a new pump with low NPSHr as the lead time of spares for these pumps in the future will not serve End user maintenance schedule.

Mr. Randal I am so happy to hear that 0 difference between NPSHr and NPSHa could be acceptable, can you explain this creteria please?

(20) George NicolaidisHow about installing a second pump in parallel? This means that the NPSHav. will be the same. you will need to lower the size of your impeller therefore giving approx half the capacity for each pump. Half capacity should mean that you are going to have a lower NPSHr per pump so you should be ok.
If you don't want to play around with the impeller size of your current pump it may be possible to use an inverter to turn its' capacity down. In conjunction with a second pump (perhaps also inverter driven) you could pick and choose on your operating point according to pumped liquid / temperatures / head etc
Are you perhaps having problems because of low ambient temperatures?

(21) Amer HamdanNo Problems with Ambient Temperature.

(22) Simon BradshawRandal is correct in that if you know your NPSHa accurately, then very low margins can be tolerated (he also urged you to seek further review, as do I). However there are a number of important caveats to this:

1) NPSHa can change as a system wears and corrodes, so what is ok when new may not be ok later.

2) At 0m margin, you already have a 3% head impairment within the pump and extensive cavitation. It takes only a very small further reduction for the pump performance to "fall off a cliff" and go into complete head breakdown.

3) Depending on the liquid characteristics and the time you operate at 0m margin, the MTBR of your pump will be reduced by some amount, possibly a lot due to high vibration and cavitation damage.

For these reasons most end users will require at least a 15% (or 1m) NPSH margin for hydrocarbons. For pumps handling water, the necessary margin for reliable operation can vary from 15% to over 100% depending on the conditions of service.

(23) Bo DixonAmer, if a booster pump is not "acceptable" by your client, "not being able to pump the EDC and Caustic" will certainly be LESS acceptable. Running 2 pumps, in series, may be a solution. Your first pump needs to be a slower running pump, with a lower NPSHr than your current NPSHa. Then that first pump can feed a second pump that will complete your discharge requirements.
If your customer WILL NOT elevate the tank or possibly keep a higher level of fluid in the tank, both creating a higher NPSHa, then try try the 2 pump approach.

(24) Amer HamdanWell, thank you all for your advice. it was realy interesting discussion. OEM accepted to design an impeller with respect to pump casing to come over this issue without changing Flow rate.

(25) SL AbhyankarPeople have used Suction Inducers. But a suction inducer is like a screw ahead of the centrifugal impeller. In other words it is like series operation of a PD pump upstream of a centrifugal pump. The inducer generates the head needed to meet the NPSHr of the centrifugal (and maybe some margin also). Since the inducer also develops head, say 3m, 6m, whatever, on the same Q, it demands additional input power proportionate to Q*dH

Before thinking of using an inducer one needs to check whether the motor has as much margin in power.

Also since inducer is as good as a PD pump upstream, and since PD pumps are fixed discharge designs, (in turn they become custom-designed device) the inducer may not help for off-design operations.

Furthermore, inducer as a PD pump will have limitations of operating speed.

Inducer is an option but not a straightforward option.

(26) SL AbhyankarAnother option is to install the pump down, providing static head which will provide margin over NPSHr.

Most installations of LPG pumps and pumps for condensate extraction are "constructed" with margin over NPSHr inbuilt in pump-design. These pumps are typically vertical pumps with pump-assembly fabricated into a can.

(27) Manish Hingrajia, Germany • Hello Amer, u can increase suction pipe dia to increase NPSH a, this will reduce suction losses, if possible lower pump elevation w.r.t suction vessel or increase vessel elevation. If possible from design point of view u can add inducer to the pump to reduce NPSHr. In reade suv. Vessel pressure.

(28) Johnny Qing Sheng Ke, Canada • Amer, 0.6 meter is the number I used in my water pumping design. However, NPSHa-NPSHr can be less than 0.6 meters in your case, since you are dealing with EDC and NaOH solutions, both have higher boiling points than water does, which is the testing media used by the manufacturer to measure the NPSHr. The nature of the NPSH is to avoid excessive air escaping from the liquid, which leads to the collapse of the tiny air bubble at the impeller surface (i.e cavitation). So you can relax the 0.6 m to 0.3 m or even lower, if you know the temperature of the liquid is no too high compare to the manufacturer's testing water temperature.

(29) Sina Sanjari, Vancouver, Canada • select vertical barrel type instead of horizontal if the differences is high,in less values delte the strainer at suction(if it is possible) and choose the bigger size of suction line.

check the curve and analyze it ,inducer can be used but check the curve firstly.

there many many case study in this case which you can easily find them in web.

(30) Chip Prybylski, Chicago • You might be able to add an inducer to the pump. This sometimes has the affect of lowering NPSHr by about 30-50% in some instances. Other than that, you will have to modify the system

(31) Amer HamdanNow it is solved , thank you all

(32) Mike Lombard, Denver, USA • Randall- From what I recall, the use of inducers will narrow the operating range of some pumps, i.e. mechanical issues or cavitation occuring if pump is throttled back or operated close to run-out? I've been out of the biz for a little while but I recall inducers sometimes causing problems in these areas.

(33) Sudhir KulkarniYou may try de-congesting the impeller eye by cutting alternate vane at suction eye. This will reduce NPSH-R, to some extent. Of course, you will have to consultant pump designer for this step.

(34) SL AbhyankarAmer, Congrats that the problem is solved ! Or is it that you had too many comments ?! :-(

But if the problem is really solved, can you please share, what the solution has been.

By the way, I am skeptical of suggestion of Mr. Randal that one can work even with zero margin. Mr. Simon Bradshaw was more correct in saying small margins can work. But NPSH-margins is an issue, which prompted Hydraulic Institute to have the standard HI/ANSI-9.6.1

I wonder whether your problem can get any guideline from this standard.

(35)

Randal Ferman Mike Lombard, I agree with your comment about inducers narrowing the operating range. And, ‘yes’, the application of inducers must be limited to sizes and/or energy levels for which the higher levels of unsteady radial loads from flow recirculation and cavitation are mechanically tolerable.

SL, This discussion thread begins with negative NPSH margin, so perhaps a typical NPSH margin simply may not be available. The statement that 0.0 m of margin is possible only serves to indicate the boundary from which an acceptable solution can be determined. For the reasons that Simon mentions, 0.0 m is probably not acceptable.

(36) Mike Lombard Thanks Randall. BTW, I recall from my Johnston Pump days, the use, on suitable applications, of a low NPSHr 1st stage impeller on multi-stage vertical turbines.

(37) SL Abhyankar @ Randal - Right, "..thread begins with negative NPSH margin..". So, what is not available needs to be made available. And the options typically are - (1) Improve NPSHa by modifying the system, there again different options such as (a) lowering the pump below the suction-line, (b) increasing suction pipe size, (c) eliminating any unwarranted bends, etc. (2) adding an inducer (3) having the pump redesigned to less NPSHr (4) Replacing the pump with one of less NPSHr.

On Option (4), in a cooling water system in a steel mill, there were four pumps running in parallel. All pumps were rated for same Q-H. Three pumps were high specific speed (2900 rpm) design and one was less specific speed (1450 rpm) design. The 2900-rpm pumps had high NPSHr, 12.5m at given Q. The plant engineer used to replace impellers every third month ! The 1450-rpm running in the same system had no cavitation-damage problem. Its NPSHr only 7m at given Q.

I was skeptical and still am, about your mention of zero margin. I mentioned HI/ANSI 9.6.1 for "how much margin".

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