Recommended Pump Suction and Discharge Side Velocities

[Pavan Kumar]

Hi All,

I wanted to get hold of a standard reference which mentions the recommended velocities on pump suction and discharge sides for a centrifugal pump. I understand the on the pump suction side NPSHA governs the velocity but a general guideline and reference would be very helpful. One of the online reference, link copied below, mentions the recommended pump suction and discharge velocities as 3-6 ft/sec and 9-12 ft/sec respectively. Norsok standard P-001 Rev 3 mentions the maximum velocities which look more erosional velocities. I am looking for more like a company standard or some good engineering practice document that mentions this.

https://www.pumpfundamentals.com/ce...e will initially,energy cost will be greater.

Thanks and Regards,
Pavan Kumar

 

[Artisi]

LittleInch, I fully agree, it's an out-of-hand pedantic round-about - - just give ALL the detail to a reputable pump company and ask for a pump selection.

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.)

 

[Pavan Kumar]

Hi georgeverghese,

L/D values should not change if use Crane values or derived values from Darby3-K method. I agree the K values estimated by Hooper's 2K method or Darby's 3K method will be higher than that estimated using Crane equivalent lengths for low Reynolds numbers.

The gate valve I am using is a full bore valve not a reduced seat one. I will remove the pipe exit as the suction pipe exits to the pump suction nozzle.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

You've used a liquid density of 66.6 lb/ft3 = 1066kg/m3 while I assumed it is 1000kg/m3, so this may be another reason for the minor discrepancies between your values and mine. Good you've got a full bore gate valve.

Given that liquid viscosity is 10cP, check with pump vendor what effect this has on NPSHr. Vendors do NPSHr tests on water with much lower viscosity, and NPSHr will be higher with higher viscosity.

 

[Pavan Kumar]

Hi LittleInch, Artisi, georgeverghese

Yes I do agree I have unduly assumed and made my system unduly complex. Since I am involved in routing the suction and discharge piping I will make sure the suction piping does not have not more than two 90 Elbows and one 45 deg elbow and try to have as short length as possible.

75 gpm flow is not a fixed requirement. Flow more or less than 75 gpm is not going to create any issue to the process. If the pump curve and system curve intersection dictates a flow of 85 gpm it will be ok. My question has been if I have been estimating the system resistance correctly using equivalent lengths for fittings from Crane TP410. The selection of the pump by the vendor depends on my hydraulic calculations. The motor is sized for end of curve, but I do not want the pump to operate at more than 110% of BEP flow rate to minimize vibrations and pump damage.

I will check with the vendor how much the NPSHr will increase due my 10cP viscosity.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

There is a detailed narrative on K, L/D values for piping elbows in Perry.

 

[goutam_freelance]

From Piping Handbook-S Crocker (M Nair):

"for steel pipe, a velocity of 7 to 10 ft/sec (2.1 to 3 m/sec) is the recommended range, while velocities to 30
ft/sec (9.1 m/sec) may be acceptable. Higher velocities are acceptable if materials less susceptible to erosion (e.g., stainless steel) are selected."

So erosion is a factor in selecting max velocity in pipe.

Regarding Suction pipe, in addition to NPSHA requirement we sometimes need to consider sufficient velocity such that dirt particles are kept in suspension. We used a velocity minimum around 1 m/s for this purpose.