Pumping control system - Auto start - Discharge pressure Trip

[MABM]

Hi,

As part of the auto-start system for two identical pumps, We would like to implement a timer to trip both pumps if pressure in the discharge line is not restored when it expires.

What would be an appropriate length to set this timer?

Pump general details:

- Centrifugal pump
- Between Bearings design
- 2 stages
- Suction presure: 20 psig
- Discharge(normal): 450 pgig
- Flow (normal): 470 gpm
- Process temp: 450F

Thanks in advance,

 

[itdepends]

What are you trying to protect/prevent?

Preventing damage to a shaft seal on the pump if run dry? (can be slow e.g. gland seal with seal water turned on- minutes is fine) to fast (mechanical seal, no seal water applied -too late it's broken).

Preventing damage/failure due to extended operation of the pump under dead head conditions (erosion- days)

Preventing catastrophic failure of the pump casing due to operation of the pump (with fluid) and closed suction/discharge valves - can calculate based on pump power input, heat capacity and volume of pump.

Something else?

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[LittleInch]

Also what are you pumping into? How long could it take to pressurize if the line is "flat" , i.e. at low pressure. Pumping into a long pipeline could take 5 minutes to see a big rise in pressure, pumping into a short pipe with flow control on the end might take 5 seconds.

As IT says, depends on what the consequences of low pressure are which determine your timer requirements. My guess is anywhere from 15 seconds to 5 minutes

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

A lot depends on what you have downstream, and also on start permissives you have on these pumps, automated inlet / exit valves, soft start, ramp up rate limited recycle control valves, level / flow controllers that operate the pump VSD etc.
If this is difficult to estimate, a coarse resolution dynamic simulation will help.

 

[MABM]

Thanks all for the advice, its appreciated.

 

[someguy79]

Recommended time between restarts could have nothing to do with the pump and hydraulics also. Often, the frequency of starts will be determined by motor characteristics such as instulation and allowable temperature rise.

 

[BigInch]

With that process temp, you're probably not running long pipelines, so pumping into a closed system, pipes nearly full, I'd bet 10 to 20 sec would be sufficient to allow normal pressure up. Allow for additional fill up time, if pipes were empty.

 

[LittleInch]

Could be anything.

So MABM - what time did you chooses and why. Time for a bit more feedback.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.