11 replies to this topic

[Ghasem.Bashiri]

Dear All,
I have a open loop cooling water system with 5+1 Pump (5 in operation and 1 as standby). During HAZOP it was recommended to add control valve at discharge of pumps (common header) to protect pumps motor for over speed during one pump failre. I have not CV in outlet of Cooling water pump. But I have a Flow element (RO) at cooling water return line to cooling tower top section.
Size of Cooling Water header is 30 inch and I think using CV for this header may not good solution.
Is there any good practice for this issue to control CW pumps outlet?
Ghasem Bashiri

[fallah]

Dear All,
I have a open loop cooling water system with 5+1 Pump (5 in operation and 1 as standby). During HAZOP it was recommended to add control valve at discharge of pumps (common header) to protect pumps motor for over speed during one pump failre. I have not CV in outlet of Cooling water pump. But I have a Flow element (RO) at cooling water return line to cooling tower top section.
Size of Cooling Water header is 30 inch and I think using CV for this header may not good solution.
Is there any good practice for this issue to control CW pumps outlet?
Ghasem Bashiri

Hello Ghasem,

You mean the mentioned pump is VFD type?

[SSWBoy]

Use of either VFD or Control Valves in a CW circuit seems a little unnecessary to me.

Once the system is initially balanced during startup there is little/no reason to adjust, use of RO's/Butterfly valves to balance users is typically practiced. The last project I looked had a 180" CW return line, I cannot imagine having a CV to control flow... can you?

[fallah]

Use of either VFD or Control Valves in a CW circuit seems a little unnecessary to me.

Once the system is initially balanced during startup there is little/no reason to adjust, use of RO's/Butterfly valves to balance users is typically practiced. The last project I looked had a 180" CW return line, I cannot imagine having a CV to control flow... can you?

I agree with the above statement regarding the necessity of CV in cooling water circuit.

Indeed,the pump motor should be protected against overspeed by its own protection system.Flow element in cooling water return line (mentioned in first post by Ghasem Bashiri) is probably installed (in combination with a FT) in order to measuring the flow and may an low flow alarm.

[Ghasem.Bashiri]

Thank you and in order to close this issue with suitable conclusion for other uses:
Consider coolng water pump with return to cooling tower. I think some restriction is required within pump discharge and before cooling tower inlet.
We used RO at inlet of cooling tower. But I should check necessity of CV installation in CW pump discharge.
I am not familiar with overspeed protection also within pump.
But failur of one pump can damage other pumps.
Now, it is practical to use CV in CW pump outlet? RO (is sufficient and practical at CW pump outlet but near Cooling tower inlet)?
Ghasem Bashiri

[ELEMAN]

Hello everybody:

Ghasem; If there is not any problem, can you show us the arrangement of your cooling system?

Thanks.

[djack77494]

Ghasem,
You call this an "open loop" system but then state that the return is to a cooling tower. I think of an open loop as being from a river (for example), through the equipment to be cooled, and back to the river, presumably downstream of the supply. If possible, and I don't see why it wouldn't be, I'd ensure that the pump motors could handle "end of curve" conditions - i.e. maximum pump flow. Then it would not be possible to burn out the motors. Unless there are some other pertinent facts that you haven't disclosed, I see no need for a control valve or restriction orifice anywhere in the circuit. And, if you don't need one then don't have one.

[SSWBoy]

Ghasem,
You call this an "open loop" system but then state that the return is to a cooling tower. I think of an open loop as being from a river (for example), through the equipment to be cooled, and back to the river, presumably downstream of the supply. If possible, and I don't see why it wouldn't be, I'd ensure that the pump motors could handle "end of curve" conditions - i.e. maximum pump flow. Then it would not be possible to burn out the motors. Unless there are some other pertinent facts that you haven't disclosed, I see no need for a control valve or restriction orifice anywhere in the circuit. And, if you don't need one then don't have one.

Types of Cooling Water

1) Once-through (the one you are describing) - where you take water from river use it and then discharge
2) Closed cycle - Closed CW system, rejection of the heat from CW is achieved against say seawater or river water (i.e. lots of plate and frame exchangers)
3) Open cycle - Rejection of the heat from CW is directly achieved using cooling tower

[Ghasem.Bashiri]

Dear All
I will email an P&ID that explain my system in detail Next Saturday.
Ghasem Bashiri

[Ghasem.Bashiri]

Dear all
Refer to attachment and my post.
I am waitng for new post as reply to me.
Ghasem Bashiri

Attached Files

•  File1.doc   196KB   51 downloads

[fallah]

Dear all
Refer to attachment and my post.
I am waitng for new post as reply to me.
Ghasem Bashiri

Do you mean OFE-006? If so,seems it is installed in combination with OFIT-006 in order to have High and Low alarms on return flow.

[djack77494]

Is there any good practice for this issue to control CW pumps outlet?

Ghasem,
This simple answer to your question remains, "Yes, don't do it". Your sketch has helped enormously in your readers' visualization of your system. It did not result in my feeling anything different about my earlier reply. Fallah and SSWBoy have provided you with good advice. Think about what you want to achieve. Control of the Cooling Water Pumps? Why wouldn't they flow at full capacity? The pump motors should not need overspeed protection. (BTW, what do you mean by overspeed protection? Modern industrial motors will run at a speed determined by the frequency of the electric power fed to them. They will not "overspeed".) While motors could overheat, if they are properly matched to their loads this should not be a problem. Also, all but very small motors should have built-in protection that would guard the motors against this.

Edited by djack77494, 30 August 2010 - 02:46 PM.