NPSHA calculation assistance 2

[Mous1747]

Hello, in the attachment i hand sketched the current system where i am trying to determine the new NPSHA
currently the 80" water level in the surge tank is providing enough NPSA to the deepwell pump.

we are in the process of replacing the surge tank, so we have to bypass it for the duration of the project which may take a week.
so basically we will relay on the 160" static head in the MGT tank to operate the deepwell pump during this week. ( dotted line 1)

I was hoping to bypass the whole surge system ( dotted line 1 )
but unfortunately i can't bypass the filters so i have to go with dotted line 2

how do i calculate the NPSHA in this case? do i just use the reading of the pressure gauge downstream of the filters ?
I only done NPSHA for simple systems but not really sure how to do that when there's a pump between the tank and my final downstream pump

Thanks in advance.

 

[pierreick]

HI,
To answer to your query : NPSH available = hs-hl+ha-hv
with : hs :static head suction line in feet or meter
hl :head losses suction line ( entrance , fittings and friction) in feet or meter
ha :absolute pressure at the surface of the liquid in feet or meter
hv :vapor pressure of the liquid at the temperature of the process in feet or meter .

Note : ha is 10.33 m of liquid , hv is negligeable (water @ ambiant) , you should not have a problem with cavitation , check the NPSH required of the pump .
Yes , You can use the Pressure gauge at the suction of the pump , if close enough to the pump to approximate the NPSH available, expressed in feet or meter of liquid .
P gauge should be converted in absolute pressure prior to convert in height of liquid.
Good luck .
Pierre

 

[LittleInch]

Curious set up you have here.

So does ALL the flow normally go via this pump and filter? Or is it a recirc unit to clean contents of the surge tank?

What is the purpose of the surge tank (I know this isn't your query, but we like to be curious here).

If the booster pump is still operating then I don't see you having any issues with NPSHA.

If not then it will depend on your friction losses through he filter which tend to vary over time as they get more clogged.

You have 3.8m + ~10m atm pressure so 14m. Less friction and PD across the filters. Unless you're>50C vapour proessure should be almost negligible.

So what's your NPSHR?



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

 

[Mous1747]

Yes all the flow go via the pump and filters, it's an environmental requirement to use the filters before pumping wastewater into the ground.

the only purpose of the surge tank is to give enough static head to the deepwell pump so it won't cavitate. otherwise there's no process or environmental requirements to have it.

yes the surge tank booster pump will always operate to push the water up into the surge tank. but they would like to get rid of the booster pump as well if possible
Why if the booster pump keep operating i won't have a problem ? How do i make sure of that ?

currently with the surge tank in place the suction pressure gauge to the deepwell pump is reading 2.3 psi so basically my current NPSHA is 1.6 meter. (the gauge is very close to pump suction)

so basically if we eliminate the booster pump, the NPSHA = 14 + negligible - piping losses - DP filter A - DP filter B ?

but what if the booster pump stays ? what would be the NPSHA in this case?

i don't have the NPSHR of the deepwell pump yet but i am looking for it. please see attachment.

Thanks in advance.

 

[LittleInch]

2.3 psig or psia?

I strongly suspect it is 2.3 psig, so your NPSHA is actually about 11.6m.

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

 

[HTURKAK]

Dear Mous1747 (Mechanical),

This thread implies the idiom "The devil is in the details"...


-If the pumping rate of the injection pump is fairly constant,filter capacity should match to this load and NPSHA calculation straight forward..

- If the injection pump pumping rate fluctuates significantly over a short period, the surge tank will 'average' the filter capacity to compensate the max .demand of injection pump. That is, the surge tank is literally a 'BUFFER' tank..in this case , when SURGE TANK is not available, the NPSHA will also fluctuate and suction pipe could experience negative NPSHA

- If the injection pump operates interruptive, probably the pumping rate of the injection pump will be grater than filter capacity and suction pipe again could experience negative NPSHA due to absence of the buffer tank ..


So,

If you want a valuable , clear respond, pls provide ( injection pump pumping rate, booster pump characteristics , filter data ; filter capacity vs head loss)

 

[LittleInch]

In the system you've just drawn then you need the booster pump to work and then it should all be good.

There's no mention of lengths here so it's difficult to see why the "surge" tank was ever needed unless your pump needs quite a high NPSH.

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

 

[Mous1747]

Attached is the injection pump curve. at the design condition NPSHA is 10 feet however the pump is operating at a lower operating point ( not sure what is it exactly yet)

So which NPSHA should i consider ? the design or the operating ?

If i keep the booster pump operating to achieve more head, however the booster pump is a different size pump than the injection pump, so capacities are different.

Now when the 2 pumps are connected in series but each have different flow, will this cause a problem?

still digging more data on booster pump and filters.

 

[HTURKAK]

There is no attached injection pump curve .


- If capacity of injection pump is greater than booster pump ,you may get problem..

- I will suggest you to provide temporary surge tank during replacement operation...

 

[LittleInch]

Mous - if the pumps are connected in series, the flow will be the same. The question is what differential head is being achieved and will the second pump pull the first pump beyond the end of its curve.

So we need both pump curves to give you an answer.

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

 

[pierreick]

Hi,
To me this post is going no where! I believe you need to stay safe to prevent the damage of your pumps , piping , etc

https://www.ksb.com/centrifugal-pump-lexicon/series-operation/191658/

You should consider to keep the same set up meaning you need to install a surge tank (temporary / 2nd hand ) and connect your pipes , filter, pump as today .
my view
Pierre

 

[Mous1747]

I finally ran all or most of the calcs based on the information that is available to me

The NPSHR for the Injection pump is 10 feet attached is the curve. and it's currently operating at 400 psi discharge pressure and 60 GPM

If i eliminate the surge tank and the booster pump and the filters (set of 5 filters )and just keep the witch's hat screens. the NPSHA is like 22 ft however this option is not possible for environmental reasons, basically we can't inject dirt underground.

If i eliminate the surge tank and booster pump only but keep all filtrations, this failed big time, in fact the NPSHA was negative

The third option was to eliminate surge tank only but keep the booster pump and filtrations, the NPSHA was well above the required. however i don't have any controls between the two pumps. so basically the injection pump is relying on the booster pump discharge conditions. is think this option maybe doable but if i have the right controls in place.

@LittleInch, that's what i am afraid of, pulling the pump outside the curve.

@HTURKAK i agree, without controls the fluctuations will damage both pumps and the piping in between

@pierreick: i agree, just replacing the tank is the best option