5 replies to this topic

[bobbobs]

Hi all - There is a tower overhead stream of vapor entering a condenser (quite a bit of elevation above grade), condensing and then going down to a set of pumps. The piping between the exchanger and pump looks like this: exits exchanger as 8", expands to 20", then back to 8" to enter pump. There is a note by the 24" section that says to locate the pumps below the exchangers and to route the 20" vertical run to maximize the liquid holdup capacity of the pumps? I was wondering if anyone could explain what this means to me. Thanks!

[kkala]

Hi all - There is a tower overhead stream of vapor entering a condenser (quite a bit of elevation above grade), condensing and then going down to a set of pumps. The piping between the exchanger and pump looks like this: exits exchanger as 8", expands to 20", then back to 8" to enter pump. There is a note by the 24" section that says to locate the pumps below the exchangers and to route the 20" vertical run to maximize the liquid holdup capacity of the pumps? I was wondering if anyone could explain what this means to me. Thanks!

Apparently a buffer drum at pumps suction is needed, to reduce consequences of flow variation that could cause even pump cavitation. Instead of placing such a buffer drum, 8" pipe has increased to 20" and then again to 8". So pipe section of height H and dia of 20" makes the required buffer tank to smooth flow to pumps. The note actually advises to maximize H, thus liquid hold up volume of said drum.

Edited by kkala, 18 April 2010 - 08:59 AM.

[bobbobs]

kkala - thanks for that. How does a "buffer drum" prevent flow variation?

[Zauberberg]

Consequently there is larger volume of liquid contained in the 20" pipe than in the 8" pipe. It's almost like a small vessele between the condenser and the pump.

[kkala]

kkala - thanks for that. How does a "buffer drum" prevent flow variation?

Suppose that H=5 m, the tube of 20" is half full (that is h=2.5 m) and suddenly flow from condenser increases by 1 m3/h.
Volume of buffer drum is about 1 m3, so it can accept increased flow for 30 min before the liquid approaches condenser. If pump flow increases by 1 m3/h, liquid level of buffer drum will come to pump suction (risk of cavitation) after 30 min.
If first disturbance occurs for 20 min and next for 10 min, operation will not be disturbed; only liquid level will be some 2.5*(20/30-10/30)= 0.83 m higher, that is h=3.33 m.
In this way flow disturbances can offset each other. Result is more effective if flow is regulated by some level control (e.g. LCV at pump discharge linked with LIC at the buffer tank of H=5m dia=20").

[kkala]

The tank (or drum) used to "absorb" flow variations (like the vertical pipe of 20" dia mentioned) is called buffer tank or equalization tank or surge tank, as one can see in literature.