9 replies to this topic

[ogpprocessing]

Dear CHE Experts,

As you know discharge side of the centrifugal pumps are designed for pump shut off pressure and that is why no PSV is installed at discharge of these pumps.

I have seen at some P&IDs that PSHH (Pressure Switch High High) is considered for centrifugal pumps.

I want to know is it necessary to consider PSHH? If so, what is the justification?

[Padmakar Katre]

Dear,
PSHH logic is normally found in the system where more than one pump is running and standby pump is on auto-start i.e. nothing but wherever the possibility of the changes in flow is there. In batch plants these applications are found quite usually where the PSHH logic will trip the pump, when the required batch of material is taken and the valve at the user end is closed(pump is running at shut-off).
Another reason is when you have minimum circulation valve normally closed and will open once the PSHH is observed.
So I can conclude that it's system specific. Pumps have minimum circulations through,
1. Forward flow control i.e. min circulation flow control valve will open/close (here either a ON-OFF valve or a control valve which will continuously monitor/control the flow.
2.ARV- Automatic circulation valves in case high head pumps, major applications in boiler feed water pumps
3.Through a restriction orifice in minimum circulation line.
So it's not a thumb rule to have PSHH on every pump system unless there is some logic behind this configuration. Below is good link to go through

http://www.controlgl...zation1003.html

I hope it can help you.

[fallah]

I have seen at some P&IDs that PSHH (Pressure Switch High High) is considered for centrifugal pumps.

Please attach one of those PIDs and send by your next post for better assistance.

[ogpprocessing]

I have seen at some P&IDs that PSHH (Pressure Switch High High) is considered for centrifugal pumps.

Please attach one of those PIDs and send by your next post for better assistance.

I have atttached one sample P&ID.

Attached Files

•  PSHH at Pump Discharge.pdf   50.08KB   78 downloads

[fallah]

I have atttached one sample P&ID.

IMO,if FALL-0025 causes the pump to be in shut down position,there is no need to have PSHH-0024 for shut down the pump in high high discharge pressure.The only condition causes the need to PSHH is possibility of strainer (in suction line) blockage.

[ogpprocessing]

I have atttached one sample P&ID.

IMO,if FALL-0025 causes the pump to be in shut down position,there is no need to have PSHH-0024 for shut down the pump in high high discharge pressure.The only condition causes the need to PSHH is possibility of strainer (in suction line) blockage.

If this concern is valid why has not been used for other centrifugal pumps? For most of the centrifugal pumps (if not all of them) this PSHH is not considered.

[ogpprocessing]

I have atttached one sample P&ID.

IMO,if FALL-0025 causes the pump to be in shut down position,there is no need to have PSHH-0024 for shut down the pump in high high discharge pressure.The only condition causes the need to PSHH is possibility of strainer (in suction line) blockage.

If this concern is valid why has not been used for other centrifugal pumps? For most of the centrifugal pumps (if not all of them) this PSHH is not considered.

[fallah]

If this concern is valid why has not been used for other centrifugal pumps? For most of the centrifugal pumps (if not all of them) this PSHH is not considered.

Each pumping system has its own concerns.

The architecture of the control system of your process depends on many parameters such as design criteria,client requests,.....

In many systems i have ever seen the pump would become in shut down position by FALL in minimum flow control valve instead PSHH in discharge line.

[ogpprocessing]

If this concern is valid why has not been used for other centrifugal pumps? For most of the centrifugal pumps (if not all of them) this PSHH is not considered.

Each pumping system has its own concerns.

The architecture of the control system of your process depends on many parameters such as design criteria,client requests,.....

In many systems i have ever seen the pump would become in shut down position by FALL in minimum flow control valve instead PSHH in discharge line.

In my view these are spurious trips and should be avoided during process design.

[ogpprocessing]

Dear,
PSHH logic is normally found in the system where more than one pump is running and standby pump is on auto-start i.e. nothing but wherever the possibility of the changes in flow is there. In batch plants these applications are found quite usually where the PSHH logic will trip the pump, when the required batch of material is taken and the valve at the user end is closed(pump is running at shut-off).

Dear,

How this auto-start logic works for live and stand by pumps? For which application the pumps should be specified ad auto-start?