Pump Curve Capacity 101

[mpeck1982]

Gentlemen,

It has been a long time since I have dealt with pump curves. I have a pump that has a 7.375" diameter impellar, 159-gpm BEP, and 187' TDH. I have also attached the pump curve. Is it possible to get 170 gpm out of this pump? Currently in the field we can't meet a 170-gpm setpoint. Another pump that is in parallel with the pump must come on to obtain that flow.

To find out where my pump is on the curve I should have inlet and discharge pressure readings in the field on the pump and that should tell me my flow from the curve correct?

 

[1gibson]

You need to define the system curve, the pump or pumps will run only at the intersection of the pump curve(s) and system curve.

If you can't hit that flow with one pump, fully open discharge valve, then there is not much you can do without major changes. Increase impeller diameter (order new impeller, verify motor HP still adequate) or try to overspeed with a VFD (can of worms, but verify NPSHA/R still acceptable, motor is VFD rated and has sufficient HP.)

If you want to know what is required, just measure the head with both pumps at 170 gpm. Create that head at that flow with a single pump, that is where it will operate.

 

[bimr]

From the pump curve, it looks like you can get 170 gpm @ 171'.

If you system head is 171' or less and you have adequate Hp, you can do it.

It appears your system head is higher than that. Do you have a discharge pressure gauge?

 

[TenPenny]

In theory, with the 7.375" impeller, you can get 170 gpm at 171 ft head with that pump. If you can't, you might want to tighten up the clearances, or better yet, put in a 7.5" impeller.

The proper solution is to measure the suction and discharge pressures so that you can figure out the system curve. In the absence of any useful data, I'd go with a bigger impeller.

You can go up to a 7.625" impeller without risk of overloading a 15 hp motor.

 

[mpeck1982]

Yes I have a discharge pressure gauge. I do not have a gauge on the suction side though.

 

[mpeck1982]

We have a 20 hp motor on the pump currently.

 

[GHartmann]

You ARE getting close to the end of the curve. The other posters are correct that pump can produce 171 TDH at 170 GPM.

Now the question for you to answer and analyze is whether that is sufficient head. You need to review the hydraulic conditions for suction and discharge systems (pressures, friction losses, height differences).

That you have to turn on the second pump implies the 171 TDH is inadequate for your system. However, 16 feet of head (6.5 psi at 1.0 SG) does not seem much change. Perhaps you can modify a control valve or some other system resistance before changing the pump impeller.

As was also suggested you DO have the option of installing a larger diameter impeller.

Be sure to take into account your liquid SG when assessing if the installed motor is adequate.

WE will give you an ATTABOY for providing the pump curve and asking the question clearly.

 

[bimr]

What is the discharge pressure?

 

[GHartmann]

One other thing, as you get to the end of the curve, the NPSHr increases somewhat exponentially (Not linearly).

The increased NPSHr can give problems, if you have a high vapor pressure liquid.

GOOD LUCK

Please let us know how it turns out

 

[Artisi]

Without splitting hairs - a quick look would indicate yes the Q and H is achievable - BUT why doesn't it currently reach 170GPM - that's the first problem to resolve? Further, there is no NPSHr or NPSHa information given which might be the answer to your problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[SNORGY]

GHartmann makes a valid point re: SG vs power - I almost posted without bearing this in mind. So, I cheated a bit and looked at was scribbled out in the title block to see if I could determine the fluid pumped. It says FLASH DRUM BOTTOMS, which might be low density and low viscosity when hot, and syrup when cold. From the curve, it gives the appearance that 20 HP might be the biggest motor the vendor would plan to put on this pump, so either they or the specifying engineer wanted it that way. For 20 HP, it's not a big cost to worry about. So, except for the uncertainty with SG, you can probably put just about any impeller that you want into this pump.

Assume for the moment - and this is a big assumption - that your suction piping is not undersized so that when you start the pump you don't see a really big drop in pump suction head at flowing conditions. The fact that you mention that you can run two pumps and successfully hit your duty point suggests to me that you are probably OK on the suction side.

To me, what is likely making this pump struggle to achieve higher flow is, as others have suggested, the system head curve. What I would be inclined to do is first read the pressure on the discharge side of the pump with the pump turned off. This should give you a good indication of the delivery pressure (hence discharge head) at zero flow. Next, read the suction pressure with the pump off, which without a gauge on the pump, should be whatever your flash drum pressure is plus the static pressure due to fluid height in the drum. In my mind, this differential pressure (thus differential head) is used to define the point where the system head curve intersects the ordinate; from this point (the vertex) you will have an approximate concave up parabola defined by (K+fL/d)*u^2/(2g). (K is to account for entrance and exit losses; call it 1.5). Irrespective of the ensuing arithmetic, what you might find is that the lower that intersection with the ordinate at zero flow becomes, the steeper will be the parabola near the duty point. What that might signify would be that you might need a significant upsize in the impeller to hit the duty point and, depending on SG, your required motor power will need to be increased accordingly. These effects will not be as significant the higher up the intersection with the ordinate becomes.

In other words, the more of the system head that is comprised of static head and pressure to overcome (as opposed to dynamic head loss), the more difficult it is going to be solve your particular problem without a different pump.

 

[LittleInch]

You can't meet 170 gpm with this pump, but what can you get out of it? What is the pressure difference when you turn the other pump on? Then you can figure out what you might need to do to get 170 out if this unit.

Can you measure flow with differential head. Yes but it is notoriously inaccurate. The curve you have looks generic so your pump might not exactly match the ideal curve. To get head you also need density at that point s well which is sometimes not easy to get. If you just have gauges instead of transmitters then you may not get the required accuracy.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[robgus85]

A few things to remember.
A pump adds energy to the system. The characteristics of your system determines how this energy is distributed between pressure and flow.
Head is independent of SG, this is why head is such a useful value.
An increase in rpm or impeller size to achieve this flow in the same system WILL result in an accompanying increase in head. Make sure your downstream piping is capable of withstanding this.
Pumps are best operated at BEP or slightly left. You are way out on the curve.

My recommendations.
Select a new pump.
OR
Reduce downstream head losses (elbows, pipe diamieter, etc.) taking into account their affect on your whole system. (i.e. dont increase a filter mesh size if you need fine filtration).

 

[DubMac]

As others have said, look at your NPSHA and note you require about 12' NPSH from the curve. I don't have a Goulds book in front of me but would guess they have optional impellers for this 3196 that may suit your C.O.S. better; check that out with Goulds.

Also, do a better job of scratching out people's names and title blocks when you post.

 

[chicopee]

Aside from what other responders have correctly written, your requirements should be so that the system curve should intersect the pump performance curve around the rated point.