Adding a Centrifugal Pump(P-1209) to an Existing Operating Centrifugal Pumps in Parallel (P-1207AB)

[Tilas]

Dear All,

We currently have our scheduled turn around soon and one of the task that we need to do is to totally empty the fresh water tank for servicing. Downstream the fresh water tank are two existing
fresh water pumps (P-1207AB) rated at 20m3/hr at 91.4 meter head. The tank volume is around 10,300 m3 thus at 20 m3/hr x two pumps running in parallel, we are expecting to totally drain the tank at around 11 days.

11 days is too long for us , thus we wanted to use another pump (P-1209A) arranged in parallel to help lessen the time for emptying the tank. The pump is also rated at 20 m3/hr at a head of 63m.
During the draining process all 3 pumps will transfer the water to an open basin (discharge valve will be throttled to have this pump operate at 20m3/hr).We plan to take the suction of the additional P-1209 from the suction strainer of the P-1207A. See attached sketch. We will totally remove the strainer of P-1207A and take the suction of pump P-1209 from there.

Performance curve also is attached for reference.

My question would be :

1. Is there any chance that the pump P-1207A starve or loose flow if we take the suction line of the additional pump P-1209 from the strainer before the pump P-1207A?
2. Is there any possible operational problem if we ran with this current configuration for the three pumps all together?

https://files.engineering.com/getfi...3-351a-4122-b6a0-7fea75ab8e2d&file=MARKUP.PNG

https://files.engineering.com/getfi...340&file=PERFORMANCE_CURVES_67-P-1207-A-B.PNG

https://files.engineering.com/getfi...b3aaa7e&file=PERFORMANCE_CURVES_67-P-1209.PNG

 

[LittleInch]

The drawing is a bit difficult to read but it looks like you have a 4" line branching to another 4 inch line before going down to 2" for the inlet into your two 1207 pumps.

You would just need to do a quick calculation to make sure when draining 40m3 down the 4" line to 1207A that you don't lower the inlet pressure below your NPSHR, but at those sizes and flows I would have thought it was unlikely.

A couple of points.

Why is the tank size so big relative to these pumps?
You need to work out how you're going to get 40 m3/hr down your discharge line when you're not raising the pump discharge level. We only have a par tof the picture here, but two 20m3/hr pumps in parallel doesn't always equal 40m3/hr unless you reduce the frictional losses somewhere.
It might be better to common up the inlet into the pump 1209 from both pump inlets so that you don't unduly unbalance the system
Even with so called identical pumps in parallel, one will pump more than the other. your pump curve is fairly flat so a few metres difference in losses or tolerances between each pump could make 5 m3/hr difference between the units and if 1+1 doesn't equal 2 then it might be even worse as you would be operating on the flatter part of the curve to the left of 20 m3/hr.

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

 

[EdStainless]

First, they are going start emptying this well in advance of the shutdown aren't they?
Secondly, is there a hatch or manway available?
You should be able to find a portable wastewater or drainage pump that you could lower into the tank that would move 40-60 m3/hr easily and then discharge through its own line.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Presume this draining operation does not require > 60m head ( the limiting head of these 3 pumps) at the required 60m3/hr. If the required head is much less, then all pumps may be running end of curve and may trip out. You may to throttle all 3pumps (individually) at the discharge to prevent them going to EOC.

What is the branch line size from P1207A suction strainer to P1209? Drawing scan resolution is poor - cannot see the detail. Pressure drop may be high for say a 3/4inch tee at 20m3/hr; have you done the pressure drop calcs? My guess is you may have to throttle P1207 more than P1209 since (a) P1209 develops less head at 20m3/hr (b) high suction pressure drop at P1209.

Why not replace the suction strainer with an equal tee (or similar) fitting for this operation if suction side dp is high? That should give you a 3inch or 2inch tee off to P1209.

 

[Artisi]

Why not hire a diesel or electric driven contractors type pump and hoses to empty the tank in a day or 2.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Tilas]

Hi Gents,

Thanks for your valuable responses.

Littleinch,

To answer some of your clarifications :

" it looks like you have a 4" line branching to another 4 inch line before going down to 2" for the inlet into your two 1207 pumps". Yes this is the case.

"You would just need to do a quick calculation to make sure when draining 40m3 down the 4" line to 1207A that you don't lower the inlet pressure below your NPSHR, but at those sizes and flows I would have thought it was unlikely." We will have at least 6.5 m of NPSHA for the flow mentioned, thus we will still have more enough NPSH margin for operation at 20 m3/hr for any of the pumps.

"It might be better to common up the inlet into the pump 1209 from both pump inlets so that you don't unduly unbalance the system." The suction strainer is the only available tapping point that we have based on the existing system , thus we are limited to tap there.

"Even with so called identical pumps in parallel, one will pump more than the other. your pump curve is fairly flat so a few meters difference in losses or tolerances between each pump could make 5 m3/hr difference between the units and if 1+1 doesn't equal 2 then it might be even worse as you would be operating on the flatter part of the curve to the left of 20 m3/hr." We don't have much of a choice as we need to use those existing pumps, we feel that for a short period of time this could be the quickest set up to drain the tanks.

But what I am worried of is the operation of P-1207A. I feel that this will not be able to give a flow near 20m3/hr as the suction of P-1209 is directly on the strainer before this pump.

Regards,
Tilas

 

[Tilas]

EdStainless,

Thank you. There should be a manhole but the tank is too big. That's why they(Operations Team) just tend to use the existing pumps for lesser work and time for arranging the loop.

Artisi,

Same as what I've mentioned to EdStainless , they wanted the easiest and quickest way to drain the tanks. Thus they do not want to use any external pump.The tank is too big and the manhole is at the top level.

Georgeverghese,

"Presume this draining operation does not require > 60m head ( the limiting head of these 3 pumps) at the required 60m3/hr. If the required head is much less, then all pumps may be running end of curve and may trip out. You may to throttle all 3pumps (individually) at the discharge to prevent them going to EOC." Yes , they will throttle the discharge valves of the pumps during this draining operation.

 

[LittleInch]

What I meant was take the inlet of 1209 from both of the strainer valves of 1207 A & B

At the sorts of differential pressure you have, loosing a metre or two on the inlet to 1207A won't make a big change.

It's good you have a flow meter D/s of 1207A/B so you have something to control against, but you will need to do the same to 1209 if your open pipe discharge means you would run off the curve.

BTW what do you mean when you say you will "totally remove the strainer" - Is this just the basket or the whole strainer??

To be honest, you could probably run your 1207 pumps at about 28-30 m3/hr each if the discharge head loss is lower than normal, but your 1209 pump no more than about 24.

Just monitor pressure, flow and max current into the motor.


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

 

[Tilas]

Thank You for your response LittleInch,

 

[electricpete]

Since the pumps are in parallel, have you verified the new pump won’t be operating close to its shutoff head?

(sorry if it’s a silly question, I didn’t spend time to study your attachments or responses closely, but the shutoff head on the new pump looked much lower than on the existing pumps)


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(2B)+(2B)' ?

 

[LittleInch]

tilas,

After waiting nearly six MONTHS to reply, I think you owe us a bit more information about what you did and what happened. Please.

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

 

[Tilas]

Hi LittleInch,

Sorry for not explaining finally what we have agreed to do.

So we decided to go as per the original plan which is to remove the suction strainer of P-1207A and connect the suction line of P-1209A there.We will operate all the 3 pumps together and try to maximize the flows while monitoring the flow, pressure and motor current (we have flow meters and pressure indicator in each pump discharge lines.

The plan is all finalized ,the temporary piping has been fabricated and just waiting for implementation during the Turn Around scheduled this coming Feb.

I will share updates after the TA.

Thanks again for the responses all of you guys.

Regards,
Tilas