4 replies to this topic

[jtodic]

I need to install pressure control valve on the bypass line of the heat exchangers. It must maintain a constant pressure at the outlet of the pumps before the heat exchangers. The problem is I do not know for which flow to design pressure control valve. Pumps will work each at 50% of system capacity. Will I design flow for:

-a minimum flow for 2 pumps - 84 m3 / h

-or nominal flow rate for one pump 230 m3 / h 

-or nominal flow rate for two pumps 460 m3 / h?

I thought to use linear type valve. The process image is attached.

 

Tnx!!!!

 

Attached Files

•  Sizing bypass valve - cooling tower.pdf   544.17KB   46 downloads

[cea]

Generally, cooling water flow, once established, is not expected to change, besides turndown or start-up conditions. Your pump hydraulic must have been done for normal condition. Prime-facie, a situation is not expected to appear where heat exchanger pressure can change significantly.

 

However, as required by you, it is required to identify situation if any (or more) of heat exchanger, having maximum flow, can be out of service at any time. One approach is to design bypass for that (those) flow.

 

Another approach is to design for one pump minimum flow, which is most appropriate, if constant pressure condition would not have been there.

[jtodic]

Thanks Cea !!!

The current system when working with single pump has too little pressure so temperature valves on heat exchangers do not work well. When working with two pumps pressure is too high. This is the reason for the pressure regulation.

[Mahdi1980]

Jtodic

As you need the pressure regulation to have a constant pressure at heat exchangers, I suggest you to suppose that only one heat exchanger is in service if applicable, so select the one which cooling water consumption is minimum among the total number of heat exchangers.

for instance the minimum flow is related to E-353 and it is 60 m3/h.

If the two pumps are spare of each other, then select just one pump for calculation however if tow pumps should be in service to feed all the heat exchangers, then consider both of them.

Then, subtract the minimum heat exchanger consumption from the normal flow rate of (one) or (Two ) pumps flow rate to find the normal flow of Bypass valve.

and subtract the minimum heat exchanger consumption of Maximum flow of pumps to find the max flow of bypass control valve.

Case 1: one pump is enough for feeding

 

230-60 (example ) = 170 m3/h normal

300 (Maximum flow of pump) -60 = 240 m3/h Max

 

Case 2: Two pumps required

460 -60 = 400 m3/h normal

550 (Maximum flow of two parallel pumps) -60 = 490 m3/h Max

[Pilesar]

I would look at this situation in terms of flow and not pressure. Keeping a constant pressure upstream of the bypass valve is not the same as keeping a constant pressure or constant flow at the temperature control valves. What flow do you need in the bypass line with both pumps in operation so that the temperature control valves at the heat exchangers are in range? You will not know this without a hydraulic study of your system. Both pumps running have a combined pump curve that is different from the single pump curve. Use the combined pump curve and the system curve to calculate the flow rate needed in the bypass. There is interaction among the variables of your system, so a computer hydraulic model would be handy. I think you will find that it is impossible for the pressure and flows at each heat exchanger to be identical between the one pump and two pump scenarios and you will have to change your criteria to achieve a successful result. Without a model, it will be difficult to show this to the project originators/approvers.