Hermetic Pump Diferential Presostat

[gwapa]

Dear Friends

What is the criterio to set the diferential presure in a diferential presostat in a Hermetic Pump?

This differentia Presostat is located between the pump discharge and pump suction pipe. It is a protection divice to evoid the pump cavitation

thanks for the comments
Gwapa

[Magoo]

Hi Gwapa.
Depends on the total pressure above vessel pressure, example pump discharge above suction at 1.5 bar gauge, set safety at 0.5 bar gauge, but time delayed for "X " seconds for cavitation and nuisance tripping at pull down and sudden loading on compressor

[gwapa]

Dear Magoo
The system has a discharge pressure 7,5 to 8 kg/cm2
The vessel 0,3 Kg/cm2

The liquid over sucction pump 1,4 m

Which is the criterio?

I adjust 10 psid (o,69 Kg/cm2) and 30 seg delay time
but whithout any criterio
Thanks
Gwapa

[Segei]

Dear Friends

What is the criterio to set the diferential presure in a diferential presostat in a Hermetic Pump?

This differentia Presostat is located between the pump discharge and pump suction pipe. It is a protection divice to evoid the pump cavitation

thanks for the comments
Gwapa
Are you sure that this is cavitation protection? Pressurestat can be used to prevent lost prime.

[RANGER1]

Each liquid pump is a little different due to m/head of pump and and static head on pump delivery when it is off .

Pressure switch could be set in between those pressures. example
normal pump pressure say 250 kpa above suction .
Static head in pump delivery line say 70 kpa when pump off .
Set switch accurately for say 125 to 150 kpa .
Control of pump can be done in a few ways oil failure type switch with say 60 sec delay shutting down pump and bring up alarm .You probably will get a lot of nuisance calls , or

PLC controlling it automatically using differential switch no delay wired into PLC .
PLC logic could be 60 sec delay on shutdown of pump if below differential set pressure. Pump to reset after 2 mins . Allow this to happen 6 times automatically before bringing up alarm .
This will allow for hiccups in plant , if it goes through all 6 resets something is probably wrong .

[gwapa]

Please se page 5 of following documents

http://www.hantech.com/documents/PDF/HP237.pdf

As you can see the HP conection is before the check valve. So when you have a buble of gas inside the pump or a loss of pressure , the static head of the pump plays a minor role

I don't find a good criterio to ajust the differential pressure

I appresiate your comments

[RANGER1]

Do you mean what pressure or how to set switch ?
Hansen instructions say 10psi differential and 30 second delay which is what you stated , so not sure what you mean by "criterio".
If you mean how to set switch , I would remove both fittings and use nitrogen with accurate gauge .
Switch to open on fall of pressure , 10psi .

[Segei]

I think that information about differential pressurestat is slightly misleading. It doesn't protect from light cavitation. It protects from severe cavitation or dry operation or lost prime. All these condition describe one even - pump isn't pump liquid. Their graph is slightly misleading as well. Unrestricted flow curve should end at 32 psig. They just put the line to show the limit of pump operation. This pump can not pump more that 53 USG. At greater capacity it will lose prime. So for this pump you can set deferential at pressure less than 32 psig.

[RANGER1]

Segei , can't say I have ever seen a switch set that high .
Would have thought a lot of nuisance trips would occur .
Of course it depends if it is a stable plant or not . things would have to be pretty smooth .

[gwapa]

RANGER1

Hansen instructions say 10psi differential and 30 second delay .

where does this figures come from?

[gwapa]

Segei
We are working with Q-Max Orifice in order to have a NPSHr less thant 1,5 ft.
With the Q-Max orifice the pump will delivery a maximo 38 GPM at 0 psi (head).

The question is:
How much differential pressure you would ajust the differential pressostat? What will be the criterio?
thanks
gwapa

[RANGER1]

Only Hansen would know .
Its a minimum pressure to make sure bearings are fed correct amount .

[gwapa]

RANGER1
Thanks for your help
Gwapa

[Magoo]

Gwapa.
you stated a max NPSH of 1.5 feet. WRONG. the pump will cavitate and die really quickly.
Select a given flow rate at design conditions, the pump curves will dictate the minimum NPSH, and the max flow and min flow rates. Add anti-cavitation plates and control compressor load rates to minimum to avoid agitated head on pump.
Once pump cavitates the bearings deteriorate big time.

[jaysephus]

personal stuff sorry

[gwapa]

Hi Magoo
You are right I should write 1,5 m insted of 1,5 feet

jaysephus

Thanks for your advise. We are in start up process and we have found some problems. The sundyne pumps What type of pump are those??

Let me tell you the last thing.
As soon all evaporators stop (Close the feeding valve)
The pump goes out. I have to check if
the minimun flow orifice size is ok .Could be too small
What do you think?
Thanks GWAPA

[jaysephus]

personal stuff sorry