Head pressure from tanks

[Isfakul]

Hi,
I have an underground arrangement of 3 setteler tanks and i want to determine NPSHA as the pump is cavitating fast.
What would be the best way to determine the head from the 3 tanks?

 

[LittleInch]

Lowest possible level which looks like 2388.

So if your pump centre line is 2378 then basically no static head.

Unless your inlet pipe is very long I can only guess that your pipe is clogged as you shouldn't be close to NPSH limit unless you are at a high temperature.

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Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

We should also ask the discharge pressure of the pumps. With little back pressure, the pump operates far to the right on the curve which puts it in the cavitation zone. An orifice plate may be a much less costly solution than elevating your tanks.

 

[1503-44]

2386
It's worse.
-2

An orifice plate will increase head, but sacrifice flow rate.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[desertfox]

I would check the NSPHr which should be given by the pump manufacturer and that would be first step, if you haven’t got the minimum suction pressure recommended by manufacturer then the installer as some questions to answer.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Isfakul]

My Npshr is 5.5m for my reqired flow rate and the pump performance is around 244m at 90l/s.
Issue is i want to install an automatic backwash strainer in the pump suction line as we often find shotcrete fibers/small rocks getting lodged into the impeller. But i am not too confident if having a filter before the pump will provide enough NPSHr to it. Will suitable a booster pump help?

 

[desertfox]

Hi

Unless I am mistaken you haven’t enough NPSHr now or have I missed something?



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Isfakul]

Hi @desertfox,
Apologies for confusion, i am concerned if i will have enough NPSHr after i install the backwash filter before the pump inlet.
The pump requires 5.5m head for reqired flowrate.

 

[LittleInch]

You almost certainly don't have enough pressure for backwash filters which need pressure higher than atmospheric to work to back wash the filters, Never mind the pumps.

Filters in inlet lines are always a bad idea when you have very little head available.

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

 

[Isfakul]

Hi @littleinch, the only reason for the backwash filter is for the Amount of solids getting lodged in the pump and having to rebuild the pump every year.
A booster pump before the filter would solve the issue I think?

 

[LittleInch]

Yes. But then the booster pump would get clogged??

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

 

[Isfakul]

Hi @littleinch that is true, but thr main pump i am trying to protect is almost half a million, so a small priming pump would be a good trade off?

 

[1503-44]

Try to reduce discharge pressure and flow needs with an orifice plate as Tugboat mentioned and you just might get some reduction in npshr, but otherwise it's not looking good.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[desertfox]

Hi Isfakul

Just been reading that you need approximately 45psi minimum to operate a backwash filter and that’s equivalent to a head of 31.69m, you don’t appear to have anything like that available.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[LittleInch]

Why are your "settlers" allowing these rocks and fibres through?

But I agree you probably need about 5 bar discharge from your booster to make a backwash filter work which is about 20% of your current discharge pressure so not a small pump either.

Fibres are difficult to filter out in a settlement tank, but rocks / solids?

what else is the purpose of these tanks?

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

 

[Isfakul]

Hi @littleinch these dewatering underground assets are almost 40 years old and the settlers not as effective as it used to be. The settler gets its feed from few other sumps around that ore body specially during wet season.

 

[LittleInch]

Well it's your money but I would repair the tanks rather than spend money on a pump, electrics, controls etc and the change it will have on the downstream pump.

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

 

[TugboatEng]

1503, if the pump is experiencing cavitation the flow frate drops off quickly. An orifice plate can increase flow rate if the cavitation is severe.

Isfakul, where on the pump curve is 90 lps at 244 m head? Can you sacrifice some flow rate? Shifting your operating point left will reduce NPSHR and provide the pressure you need for flushing filters.

A large enough filter creates little restriction. A rubber coating on the impeller can improve resistance to damage from cavitation.

 

[1503-44]

Tug, I think you are not using the pump curve as a reference when you say "increase flow rate". So, I have to ask, increase flow rate over what? I would agree that an orifice plate could increase a flow rate that was being limited by cavitation. Otherwise my logic says that an addition of an orifice plate effects flow and pressure just like any addition to the resistance of the system curve, resulting in moving the operating point to the left; higher heads and lesser flows. Lesser flow in the system thereby tends to increase NPSH, possibly to the point where cavitation stops, but in any case maximum possible system flow, as determined by pump & system curve operating point, is reduced from the capacity it had before introduction of the orifice plate.

I would try adding an orifice plate to solve this problem on the off chance that the reduced flow rate (operating point reference) is still acceptable, however otherwise I'll agree with LittleInch in that fixing the cause of cavitation is the best technical solution, rather than placing plasters, stitches and clamps while keeping one's fingers crossed that knock-on effects can be avoided.



A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[TugboatEng]

Was there a pump curve posted previously? It seems like some of the responses have information I am not aware of.