6 replies to this topic

[star@25]

Guys Please help me.... Why do we need minimum circulation in the centrifugal pumps?

[breizh]

Guys Please help me.... Why do we need minimum circulation in the centrifugal pumps?

should answer your query
Breizh

[Zauberberg]

This one will be helpful as well: http://www.chemicalp...s/2007/109.html

[Padmakar Katre]

Dear,
Rather understanding only minimum circulation I will suggest you to get a pump handbook and just get the basics cleared and mean what pumps are.

[kkala]

Although material presented explains matter adequately, also note the following:
1. Working for a company implementing Projects in Petroleum Refineries, I can see that min flow is installed now on every pump, which was not the case some 25 years ago.
2. Automatic flow control is the usual way; an alternative is the recirculation valve (e.g. yarway) seen on the discharge of ammonia pumps or boiler feed water pumps (often given by the pump supplier).
3. Necessary minimum flow is usually 20% - 40% of rated pump flow (limits can be rarely exceeded). We size return pipe and control valve for 40% of rated flow and check it after the pump order, when pump supplier tells the min flow.
4. Min flow should return to the suction tank. Avoid return to the suction line, since recirculation will increase fluid temperature.
5.If we have to connect min flow line to the suction line, we check (conservatively, e.g. by considering all spent motor power to be given as heat) the time when fluid temperature approaches a critical value (e.g. temperature for pump cavitation), on the condition that the first valve in the discharge line is fully closed. This time should be long enough for intervention (depending on the specific case), e.g. 15-20 min, otherwise the connection point should be more distant. Besides a high temperature switch / alarm should be installed.

[JoeWong]

Guys Please help me.... Why do we need minimum circulation in the centrifugal pumps?

May be too late to come on this topic...

There are at least four (4) main factors possibly lead to requirement of pump MINIMUM RECYCLE flow. There are :

i) Fluid temperature rise
Dead headed pump results energy inputs into fluid subsequently temperature follow by vaporization and cavitation and finally damage pump

ii) Internal recirculation
Internal recirculation under low flow lead to slow heat dissipation and severe erosion corrosion.

iii) Thrust capacity
Internal thrust force reduce lead to potential damage of pump bearing.

iv) Minimum system stable flow
Interaction between system and pump lead to present of system minimum flow.

Edited by JoeWong, 17 December 2009 - 12:41 PM.

[Shahnawaz Makroo]

Dear friend please note the reasons for providing MCF in pumps as it acts as pump ptotection.

1.Pump minimum flow is more than the system flow to me pumped downstream.

There is certain minimum flow for every pump below which pump can not be operated to avoid cavitation and vibrations in shaft of the pump.e.g we need 10 m3/hr flow in system to be pumped but pump selected is having minimum assured flow of 15 m3/hr.Then we proveide minimum flow line for (15-10)= 5 m3/hr to backe inthe suction source and 10 m3/hr in the system needed at doenstream.

2.In operation suppose downstream valve shuts off because of operation reasons, that time minimum flow line is needed to avoid reaching of pump shut off pressure and hence tremperature rise in the pump casing.Therefore pump is put in to the circultion through MCF line till problem is solved in downstream.

3.During starat up of pumps in commissiong McF line is useful for pump start up till downstream system is ready to take pump in line.


Regards
Shahnawaz.