3 replies to this topic

[GMPATEL]

Regarding the hydraulics of a centrifugal pump suction, what should be the maximum allowable velocity?  The process fluid is Gasoline with a vapor pressure of 55 kPA at 20 ^oC.  The pipe is Carbon Steel, API 5L.  The flow through the pump is 135 M³/Hr. 

 

Is there any standard like API, Shell DEP, Total, BP, etc. to base maximum permissible Suction velocities.

 

Thanks in advance.

[fallah]

GMPATEL,

 

In general, lower suction velocity will provide higher NPSH available and obviously is better...

 

As per TOTAL for non boiling liquid: up to 2" size 0.9 m/s, from 3" to 6" size 1.2 m/s, from 8" to 18" size 1.5 m/s and for 20" size and more 1.8 m/s are recommended as maximum velocity in pump's suction line... 

[Art Montemayor]

Gmpatel:

 

What Fallah has stated is good, general recommendations.  But, as I am sure he will also agree, these recommendations are very general and are subject to the local conditions, piping layout, piping distance and configuration, variations in flow rates and suctions heads, and - most important of all - the Net Positive Suction Head Required (NPSHr) by your pump(s).

 

In my opinion, the fluid you propose to pump (gasoline) has a relatively high vapor pressure - even at 20 ^oC - and requires you to carefully design the suction piping with consideration to the Net Postive Suction Head Available (NPSHa).  The higher the value of NPSHa above that of the NPSHr, the better off you are and the better results of the pumping operation.  This should reinforce what Fallah has correctly stated.

 

The values given by Fallah are just general values and intended only to be a guideline with the knowledge that there are variations in their accuracy when you consider the actual, local conditions as stated above.  There is no hard and fast rule for identifying the design suction velocity for a centrifugal pump.  The NPSHr, above anything else, MUST be respected and held as the measuring bar of a successful pumping operation.  And don’t forget that with age, maintenance, wear, and other conditions the pump’s NPSHr may increase.  That is why it is always wise to be very conservative in designing the suction piping (more importantly defined as the suction resistance to flow) and take into consideration all other factors that may influence the NPSHa - for example, will your specified 20 ^oC temperature ALWAYS be constant?  Will the suction temperature increase to a summer time, exposed value of 30 ^oC?  Additonally, the practical engineering concepts also apply:  suction piping is often much shorter than the discharge piping and since a minimum of fittings and valves are used, this piping run is very inexpensive when compared to the discharge side and is usually a small percentage of the overall pump installation cost.  That is why the general rule of thumb is to make the suction piping 1 or 2 sizes larger than the discharge nozzle on the pump.  In other words, if the pump has a 2” discharge, make the suction piping run 3” or 4”, depending on local conditions.

[Zubair Exclaim]

Two criterias are there

 

a... velocity like what Falah said.

b... pressure drop per unit of length criteria. for pump suction this value should be 0.5 psi/ 100ft or lower.

 

Like what Art Montemayor said .. be very carefull about NPSHa, an keep a NPSHa margin inside calculations... some engineering designers keepa  margin of 3 to 5 ft in NPSH calculat, this margin is in addition to the 3ft margin between NPSH available and required as per API

Edited by Zubair Exclaim, 17 August 2015 - 01:31 PM.