8 replies to this topic

[-G-]

Hi folks,

 

Looking for some advice on the design of suction pipework which would be able to accommodate two different centrifugal pumps, but first let me describe our current setup.

 

We have a waste water pit approximately 4 meters deep where pH is adjusted by dosing either acid or caustic. We currently have a high spec end-suction centrifugal pump which circulates this water during neutralisation and discharges it back to the pit. When the pH is in the desired range, we switch to another valve on the discharge side which discharges the water off site. First picture attached shows our current setup.

 

Due to high cost of this pump and life span of a few years, we have acquired a cheaper self-priming centrifugal pump which we want to use in its place. However should the new pump fail, we would like to be able to switch back to the old one which will be kept as a spare. The only problem is the pumps have different suction and discharge diameters.

 

Current, non self-priming pump: Suction diameter 100mm, discharge 65mm

 

New, self-priming pump: Suction diameter 80mm, discharge 80mm

 

Our existing suction pipework is 150mm from the very bottom up to the pump suction, where it reduces down to 100mm. I have been told by the manufacturer of the new pump that you cannot go from a larger diameter down to 80mm on the suction side, it will have to be 80mm for the entire length. This rules out the possibility of keeping the current 150mm pipework.

 

So my thinking is to have flanged spool pieces on the suction and discharge for each pump to allow them to be swapped out if required. Please see second picture attached (discharge not shown).

 

Any input, thoughts or alternative ideas would be much appreciated.

 

Also with regards to the current inverted U-bend design, I’m led to believe this is to keep the pump primed if the vertical section was to drain? That line coming off at the top of the U-bend is blanked off and not sure what it was used for in the past, perhaps degassing any accumulation of air? I understand three 90 degree bends right before the suction isn’t ideal due to turbulence, but I’m reluctant to get rid of it if it’s serving a purpose.

 

Thanks

Attached Files

•  Current setup.jpg   70.09KB   4 downloads
•  Proposed changes.jpg   53.68KB   6 downloads

Edited by -G-, 29 June 2016 - 05:08 AM.

[shvet]

 I have been told by the manufacturer of the new pump that you cannot go from a larger diameter down to 80mm on the suction side, it will have to be 80mm for the entire length.

 

 

It seems manufacturer is wrong. Larger diameter of suction line is the conventional practice.

 

 

 This rules out the possibility of keeping the current 150mm pipework.

 

I would recommend use existing 150 mm.

 

 

 

Also with regards to the current inverted U-bend design, I’m led to believe this is to keep the pump primed if the vertical section was to drain? That line coming off at the top of the U-bend is blanked off and not sure what it was used for in the past, perhaps degassing any accumulation of air? I understand three 90 degree bends right before the suction isn’t ideal due to turbulence, but I’m reluctant to get rid of it if it’s serving a purpose.

 

Air pockets are not recommended. If it is serving line it does not mean it works well. You should check pump maintenance notes and ask repairman for noticed cavitation signs.

Edited by shvet, 30 June 2016 - 10:31 PM.

[breizh]

Hi G ,

To support your work , consider the doc attached .

 

Note : You should install a foot valve and a connection to prime the line  . The U bend will not prevent the  cavitation with a risk of air locked  .

 

Good luck .

 

Breizh

Edited by breizh, 30 June 2016 - 11:49 PM.

[-G-]

shvet

 

The new pump supplied by the manufacturer is a self-priming pump, supposedly they are an exception and require suction pipework of one diameter all the way. I've not looked into the technical reasons why but I've read from different sources this is the case.

 

With regards to the inverted U-bend, I've seen this in a document from the manufacturer, see first picture attached. Please note this is for our existing non-self priming centrifugal pump.

 sulzer self prime design.png   590.89KB   2 downloads

 

This pump has been replaced last year, with the old one showing signs of cavitation on the back side of the impeller, so perhaps excessive friction loss caused by these bends is causing the NPSHA to drop below vapour pressure?

 

breizh

 

Thanks for the info, I've been having a go at the NPSH calculations.

 

I think the U-bend is only there to keep the pump primed, as shown in the picture attached. A foot valve as you mentioned would do the same but I understand they can be unreliable, so wouldn't want to rely on that alone? Ideally I want rid of the U-bend, as it's proabbly contributing to the cavitation we've experienced with our last pump.

 

Anyway, back to NPSH!

 

I've been doing the calculations for the new self-priming pump which has been supplied by the manufacturer, and from what I've calculated it's not up to the job?!?! Unless I've missed something or made a mistake.

 

Can someone have a look? Word doc attached

 NPSHA for new KSB pump.docx   62.76KB   30 downloads

 

Thanks.

 

Edited by -G-, 01 July 2016 - 07:37 AM.

[shvet]

shvet

 

I've been doing the calculations for the new self-priming pump which has been supplied by the manufacturer, and from what I've calculated it's not up to the job?!?! Unless I've missed something or made a mistake.

 

Can someone have a look? Word doc attached

NPSHA for new KSB pump.docx

 

Thanks.

 

See notes in attached file

Attached Files

•  NPSHA for new KSB pump.docx   404.18KB   36 downloads

[-G-]

Shvet,
 
Many thanks for working through this. I used a design coefficient of 150 as the pipework will be PVC.
 
Yes 7.90 m is a lot higher than our current pump, with has a NPSH-R of 4m
 
Data sheet attached:
 
 New Pump Data Rev2.pdf   173.65KB   27 downloads
 
So as it stands it looks as though this pump isn't suitable for the application?
 
As for volumetric flow and velocity I need to do more reading to understand this.
 
I'm assuming if the data sheet says 27.5 l/s then it doesn't change, same with velocity as pipe cross sectional area is constant etc.

 

Edit to add:

 

I've just discussed with one of our operators that the current Sulzer pump we have in  is actually self priming. Supposedly it uses sealing water to draw a vacuum in the suction line if there's no water?

Edited by -G-, 01 July 2016 - 10:14 AM.

[breizh]

-G-

A great document from KSB to support you work.

I'm sure it will help you and others .

 

Breizh

Edited by breizh, 02 July 2016 - 04:41 AM.

[shvet]

Shvet,
 


So as it stands it looks as though this pump isn't suitable for the application?
 
 

 

I think so.

To be more accurate - KSB suggests pump which almost useless. KSB suggests pump designed for undeground pits but in fact this pump can pump out only not too deep pit.

Let's look at some details more carefully:

 

1. Suction line DN

 

You talked that suction pipe shall be the same DN that pump suction nozzle. It looks very and very strange. In your case in suction you will have velocities = 5m/s - it is very high value. Very! Velocities in discharge line usually are 1.5-2 m/s. See some recommedations for velocities, if you need more recomendation sourses - say here.

In your case only velocity head in suction line will be

Hv = v^2 / 2 * g = 5^2 / 2 * 9.81 = 1.27 m H2O

 

So high velocity leads to very high friction drop - almost 5 m H2O per 5 m of pipe. It is very high! Sounds like there is some mistake. DN of suction lines usually is larger than pump suction nozzle. See API 686 extraction in my previous post.

 

2. KSB suggestion

2.1 Actual pit deep

 

If you listen to reason and increase DN of suction line you can decrease friction drop (Hf) and velocity head (Hv) almost to 0 and neglect them. In this case your the situation becomes better but the same bad.

You should find out what is:

- safety margin for pit overfilling prevention, let's assume it = 1m

- suction nozzle datum elevation (NPSHR datum depends on pump type, see API 610 for more information), let's assume it = 0.6 m

- safety margin for NPSHR test accuracy, starting NPSHR margin and NPSHA calculation accuracy, let's assume it = 0.5 m (conventional value)

 

In this case you will have only

2.038 - 1 - 0.5 = 0.5 m of operation region - distance between min and max liquid level.

 

If your pit diameter is 3 m it is only:

0.5 * 3.14 * 3^2 / 4 = 3.5 m3 of waste water and it is 3'500 litres / 24.5 l/s = 144 sec = 2.4 min of waste water stock.

 

It seems to me as rather low value - only 2.4 min. Do you actually need pump which can pump out pit for 2.4 min? May be you should increase pumping period to 20-30 min?

 

2.2 Pump type

Imagine - you are KSB ingeneer and you design a pump for pits.

What can be max allowable deep of pit in theory - 10.3 m? Because it is atm. pressure and if pit is deeper any liquid will boil during pumping. OK, next.

Designing pump will pump out subcooled liquids, mainly water which vapor pressure is 0.3 m. It means in theory you can get

10.3 - 0.3 = 10 m

OK, next

You designed a pump and tested in on water. You got NPSHR=8 m, It means that in theory you can get

10.3 - 0.3 - 8 = 2 m

OK, next

And this value you (KSB engineer) should decrease for friction drop, velocity head, possible vortex formation, NPSHA calculation prediction accuracy, flow fluctuation, suction nozzlre datum, starting NPSHR margin,  NPSHR test accuracy and fluctuation in real pump constraction! Only 2 meters for all this factors!

Real? What? Who needs a pump which can pump out not deeper 1.5 m? Where are pits 1.5 m deep? Who will buy this pumps? And on top all of this KSB suggests to use the same DN of suction pipe with 5 m/s and 0.5 bar pressure drop per 5 m. What?

 

All this reasonable if you designed and sell a floating pump. A self primed pump which floats on platform in pit, inside, on water surface. Yes, this type of pump can have NPSHR = 8m and DN of suction line the same as suction nozzle is.

Edited by shvet, 02 July 2016 - 04:41 AM.

[-G-]

breizh, shvet,

 

Many thanks for your input on his, and apologies for not bringing this thread to a conclusion.

 

After all the calculations and discussions here, I contacted KSB and forwarded my calculations on to them. The sales guy said he would come on site and discuss the matter. Lo and behold the pump sold to us was not suitable for the application!

 

He repeated the calculations I had done but was much more forgiving on the frictional losses. He just referred to a chart that gave friction loss based on STRAIGHT pipe length, not taking into account 90 degree bends etc. He suggested we could put a throttling valve on the discharge side of the pump, lowering the throughput to reduce frictional loss and increase NPSHa. This wasn't acceptable and the pump was returned for a full refund

 

But hey at least I've learnt a thing or two about pumps

Edited by -G-, 24 November 2016 - 09:30 AM.