3 replies to this topic

[ANIND]

Hi All,

I have a system where a new centrifugal pump (taking suction from an existing Tank) has a FIC with control valve at the downstream and which discharges finally in an existing crude oil line. The new pump shutoff pressure  is higher (Class #300) than the downstream system design pressure (Class #150). Hence a relief valve needs needs to be installed (for blocked outlet case) at the discharge with a set pressure same as the downstream system design pressure to protect the downstream pipework.

 

The problem is the normal pump discharge pressure is close to the relief valve set pressure, therefore during alternate cases like turndown operation, as the pump head rises along the performance curve, the relief valve if it is installed at the immediate pump discharge is expected to pop-up frequently. Hence my understanding is that the PZV at the pump discharge should be shifted at the downstream of flow control valve while the pump discharge line including the control valve shall be designed for the pump shutoff pressure (#300).

 

Moreover Aramco standard SAES-J-601 mentions that for such system where a PZV is installed for downstream protection, a high pressure trip should also be configured at a set point slightly less than the relief valve set pressure. Typically I have seen pressure transmitters installed at the immediate pump discharge for high pressure trip. But in this specific case, I believe the PT having a set point slightly less than the relief valve set pressure should also be installed at the downstream of the control valve.

 

This is because the normal operating pressure at the pump downstream upto the control valve is near to the relief valve set pressure. If the pressure transmitter with relief valve is installed at the immediate pump discharge, any upset or a turndown operation would lead to head rise with frequent pump trip and occasionally triggering the PZV also to pop-up. Therefore I want to verify with the senior members in the group that if my location of pressure sensing element and relief valve at the downstream of control valve seems correct and to give their valuable inputs (if any). 

[fallah]

ANIND,

 

Lack of detail info: simple sketch, pump curve, downstream system route and isometrics,...

 

Anyway to resolve the problem, at a glance and for example, you might reduce the downstream operating pressure by adding a by pass around the pump to increase the flow passing through the pump (while the required flow is kept at the pump discharge) hence to shift the pump operating pump to the right side of the pump curve leading to the lower normal operating pressure at downstream.

Of course, above modification will result in more power consumption for the pump which relevant motor should handle the new power consumption...

[ANIND]

Thanks Fallah for the answer. But I think even if we bypass a certain fraction to increase the total flow through the Pump, pump discharge pressure shall be governed by the system back-pressure developed due to the forward flow which still remains the same. I hope that the required discharge cannot be altered.

 

I have attached a sketch for more clarity on my discussion point.

 

 Schematic.xls   313.5KB   23 downloads

[fallah]

ANIND,

 

Then, could you remove blocked discharge scenario from the pump discharge line, say, by considering LO (locked open) facility for all manual valves along the discharge line?