Pumping / Piping Circuit 2

[moiz19670309]

We have a water reservoir on its roof we are planning to install two vertical turbine type pumps. Each with a flow of 160 m3/hr with ahead of 75 m. We are required to deliver at a demand point a flow from zero to 300 m3/hr with a head of 65 m. The scheme is shown in the attached picture. My questions are;
- Does the scheme seems sensible
- Does a Flow control valve as shown in scheme , works the way it is shown
- Any suggestions with respect to meeting the varying flow demand with constant pressure of 65m (0.6 MPa)

I have little knowledge of such type of pumping/ piping circuits.
Urgent response is highly appreciated.

 

[LittleInch]

This really is pretty fundamental stuff.

Some questions:

Who controls the flow - I can only assume this is downstream your supply point?

What you probably need is a flow control on each pump sized to recycle the minimum continuous flow. Above that flow the pump doesn't need this return loop, but then what you will need is a pressure control valve at your connection point regulating the downstream pressure to 65m.

Your system would either waste large quantities of energy or could deliver in excess of the required head.

How you start / stop the second pump also needs to be considered so you need a flow meter on your system somewhere to bring the second unit on as you approach 160 m3/hr and turn it off as you fall below 160.

Maintaining equal flow in a parallel pump system is notoriously difficult as well so you need to specify your pumps well - check out some of the parallel pumping threads on this forum - basically ensure quite a steep head flow curve to the left of your duty point.

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[moiz19670309]

Thanks for the quick reply. Your assumption regarding usage is correct. Based on your advice i have modified the piping layout. Kindly check and provide comments.
Thanks

 

[bimr]

What is the application or use for the water?

Why is there such a big tank on the roof?

 

[moiz19670309]

LittleInch , i am waiting for your response, please.

 

[bimr]

I see it is a pump on the reservoir roof.

See the attached sketch. You only need one pressure control valve. You might want to consider a spare pump.

 

[bimr]

What is the application? Are you planning to run 24/7? Is this for irrigation or potable water?

You would get a better answer is you offered some details on the operation of the system.

http://www.singervalve.com/pressure-sustaining/106-206-rps#Tabs1

 

[LittleInch]

I'll sketch you something tomorrow.

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

 

[LittleInch]

In the mean time I suggest you look up ARV, automatic recirculation valve. That's probably what you want on the pump lines. Your preassure regulating is just a valve. No need for a return line.

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

 

[moiz19670309]

Ok , the water is to be used by Building Construction Contractor. We will use three 50% duty pumps. Peak demand is 300 m3/hr. And as you see we will have a standby pump. Based on advice by LittleInch i have modified the pumping circuit. kindly check.

 

[bimr]

I would use something like the Cla-Val Model 49-01/649-01 Rate of Flow and Pressure Reducing Valve to maintain the pressure and the Cla-Val Model 50-01/650-01 Pressure Relief Valve to return flow to the manifold.

I would not use ARV valves. An automatic recirculation valve is a multifunctional valve whose primary purpose is to ensure that a pre-determined minimum flow is assured through a centrifugal pump at all times.

See the sketch in the attachment and the valve literature

Install a pressure transmitter on the discharge line and use pressure controller to turn the pumps on/off.

If you plan to operate this for an extended period at low flows, you might consider some energy saving alternatives.

 

[LittleInch]

Bimr, I don't think we're actually too far away.

Your valve is what I mean in terms of pressure regulating the downstream pressure. The OP has stated flow control is by the downstream user and can vary 0- 300 m3/hr. Without any idea of what the actual usage and flow is over a day, we can't restrict this system to operating in one mode or the other.

There is no accumulator here so using pressure to turn pumps on or off could mean multiple starts/ stops, hence my suggestion of an ARV as the OP has stated he is using pumps which need a constant minimum flow.

I would suggest the control is done on flow - 0 to say 150 one pump, 150+ two pumps when rising flow, back to one pump at say 120 on a falling flow to prevent the second unit starting / stopping frequently. FLow less than minimum flow ( at a guess 25% of rated so 40 m3/hr will activate ARV to bleed off the remainder when required to maintain 40 through the pump.

Operation at low flow is a little wasteful on energy using ARVs, but it makes the control simple and allows rapid change of flow as someone downstream opens the valve.

Period of say 5 mins on more at 0 flow, turn the pump off?

when operating in parallel mode, check the running amps of each motor. If they're within 10% then leave it, but if a large discrepancy occurs, you could trim the larger amp motor by e.g. slightly closing a butterfly valve until then become more equal. "Identical" pumps and piping systems are rarely identical and if your pump curves are quite float, you could easily get an imbalance between units leading to overheat or tripping.

See attached - very similar to last one from the OP

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

 

[moiz19670309]

Thanks again to all valuable participants. Best regards.
I will follow the advice and if required i will ask again.
Moiz khan

 

[Softedge]

moiz19670309,

another and maybe more economical (environmentally friendly) option is to use three pumps, one each of 50, 100, and 160 cum/hr. have all three pumps on variable frequency drives. the pumps would use a downstream pressure sensor and control logic to determine the number of pumps operating and the speed at which they run. if you know the statistical mode value for the flow rate have the system start with the pump that delivers that flow rate, the logic should be able to control the number of and pump speeds from there. by this method you should not need a return loop to the tank and the associated energy waste.

softedge

 

[LittleInch]

Maybe, but I get the impression that this is a fairly rough and ready system if it's being used by a building construction contractor.

Until you actually run it I doubt you'll get any decent idea of the flow pattern over a day and how much it varies.

A couple of tips though:
1) put double NRVs on your pump outlet if you're not using actuated valves - they are notorious for leakage, but do allow simple turn on/off of your pumps.
2) tick the box or add to the pump & motor data sheet or specification that the motors should be suitable for use with VFDs. Then you can retrofit one later if it seems a good idea. will add very little if anything to the cost of the motor, but would save a lot in the future and avoid needing to change the motor out.

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

 

[Softedge]

what you say is true, however there are a couple of factors that support the recommendation for the use of vfd's and multiple pumps. yes, this could be a rough and ready system however the tank size required and the flow rates, up to 300cum/hr (1300usgpm), would indicate the system may be used repeatedly.

this is going to be a sizeable tank, pipeline and pumps. the pipeline at 300cum/hr could be 6" +/- and the pumps at 160cum/hr and 65psi are going to require a 75Hp +/- motor. moiz19670309 can respond, but this doesn't seem to be a one use system.

this may sound like, but i am not ranting with the following.

most designers and tradesmen with a knowledge of the application can throw together a system that will do the job. as applied scientists in the modern world we have a duty to recommend the most efficient systems for a given application, we can no longer afford to waste as we have in the past. the side benefit of this is that usually the more efficient system is also the most cost effective solution to the owner when considered from cradle to grave.

a healthy debate is always worthwhile.

 

[Softedge]

correction 65m and 75m at the pump outlet not 65psi.

 

[LittleInch]

Softedge,

I do generally agree, but we don't have the info to say yes or no. VFDs are not a free lunch here. They also ooze heat and inefficiency and will cost more. Providing that the flow rarely drops below the min flow rate of one of the pumps, fixed speed units might be more efficient. Even then the amount lost through min flow recycle is quite low as flow rate is not high. The total power here is really quite small.

Your line about "we can no longer afford to waste" is sadly not always true. Energy is very cheap compared to many other costs so it can take a mighty saving to translate into capital cost savings, especially if this unit is only planned for a few months or years of operation.

We will probably never know so maybe park this one here for the time being....

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

 

[bimr]

Some facts that are known is the flow rate (300 m3/hr) and discharge pressure (65 m) are high.

However, unless the OP is willing to present the details of operations so that a reasonable equipment suggestion may be offered, it is pointless to speculate.

If you were planning to operate 24/7, softedge has a good argument. If you were planning to operate 6 hours per day for a few months, then LittleInch has the best argument.

As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don’t know we don’t know.

 

[LittleInch]

Bimr - well said, though we clearly work in different industries as to me 300 m3/hr and 65m ~ 75kW is a small booster pump. Try the 1MW-5MW pumps.... Now they are a decent size.

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