2 replies to this topic

[ayan_dg]

Can I use a velocity of 6 m/s in pump min flow line (recirculation to suction vessel). The fluid is clean water at 40 degree Centigrade. If I do not use this velocity the pressure drop in the line becomes so less that I have to use a very large DP across the control valve & this is becoming very close to the critical pressure drop.

[Art Montemayor]

For clean water, I recommend the normal 5-7 ft/sec of average velocity. You should never qualify the size of line by the pressure drop you have across a control valve. If anything, it is always the opposite.

What do you mean by "critical flow"? There are many things today being defined as so-called "Critical" - and people fail to stop and define what they mean. If you are dealing with clean water, what is so critical about your flow?

If you are reluctant to take the full pressure drop across the control valve, that's OK. Simply install a 2-stage pressure reduction valve. This is what is commonly done with gas regulators when they are confronted with supersonic flow. You can also use a pressure regulator before your control valve.

[viral desai]

Before I proceed to give you an answer, let me know what is the temp and pressure at which you handle water. Do you mean that you encounter pressure in the control valve below the vap pressure of water at that temp?

Also, 6 m/s is very high velocity if you are using carbon steel as the MOC.

You should be using 6 Fps as a design velocity for the min. circulation line and size the line based on pump min. ciruculation flow.