Sizing discharge Pipe for Positive displacement pump 1

[bmw318be]

We have a metering hydraulic diaphragm type pump with design suction pressure positive, discharge pressure 13 barg.

If turn out to be the pump is 2", it is feasible option to use of 1" discharge pressure.

Assuming the pressure loss calculated has been calculated based on the 1" pipe diameter, the pump rated pressure are 14 bar.

Any impact of downsizing the discharge pipe after the pump or should i consider to reduce after passing all the accessories which would be installed after the pump ( Pressure relief valve, pulsation dampener and Back pressure valves

Appreciates your valuable feedbacks

 

[MikeHalloran]

The small discharge pipe will limit the instantaneous discharge flow rate per cycle, hence will limit the cycle rate, which in turn will limit the overall integrated flow over a long time. If it's a batch process and the pump doesn't have to run continuously to keep up, it may not be a problem.

OTOH, 2" pipe is not substantially more expensive than 1" pipe, when installation labor is included.



Mike Halloran
Pembroke Pines, FL, USA

 

[bmw318be]

Hi Mike,

For positive displacement, my understanding it has constant flow regardless the pressure, however a dosing of chemical to the water is my application, there is required accuracyof the chemical dosing.

You might be right there would be a constraint in overall cost if the pipe are of 100-150 m long on the discharge line, however most of the long piping are horizontal and not a vertical lift.

Could you elaborate instantenous flow rate, let say my rated flow with 2" are 1500 Litre per Hr. If i use 1" discharge pipe, shall i get the same 1500 LPH for positive displacement pump

 

[micalbrch]

"....or should i consider reduce after all the accessories which installed after the pump ( Pressure relief valve, pulsation dampener and Back pressure valves)?" No, definitely not. The size of these accessories is determined by the max. stroke volume and the pressure and that will not be reduced by a smaller discharge pipe.

 

[bmw318be]

Hi Mical,

Thanks for your advise, since you touch on accessories of external relief valve, back pressure valve, I would like to know if there is any impact of using smaller size of accessories connection port.
Metering pump typical P & Id

Let say we have sized the BPV and PRV and Pulsation dampener and recommended based on flow and Stroke per minute is

1" Pulsation dampener
1" Back pressure valve
1" Pressure relief valve

However my discharge pump port is 2", shall this work if i maintain the 2" port and need to use reduce adaptor for every accessories ports


So to minimize the use of adaptor, trying to standardize the discharge port where the accessories would be installed.

Any situations if this exist for installation practice for metering pump where all the accessories are being installed with 1" port size.

I have read some article that we shall not downsize more than 1 size, i am not sure the technical reason behind other than this

[QUOTEDfrom whitepaper]

https://www.google.com.sg/url?sa=t&...ggiMAE&usg=AFQjCNGQ2MqU6ngLvlLat0csETffyIarlA

Discharge Side
Discharge piping in a pump system has virtu- ally
no effect on pump performance other than the
head loss that it creates. Optimal discharge
piping design will assist in minimizing installa-
tion and operating costs. Too small of pipe sizes, fittings and accessories will cause high velocities,
increased pressures and wasted energy.
Sizing the system piping too large will increase
the initial capital cost of the pipe, fittings and

[END]

However, reading another article, i believe this is not relevant to positive displacement, metering pump

[Quoted]

http://www.seepage.com/learning-center/sump-pump-failure-your-discharge-pipe-may-be-too-small

If the head height is smaller, the pump will move a little more water per minute; if it is larger, the capacity will diminish.

More importantly, the capacity rating of the sump pump is also based on using the right-sized discharge pipe. Most sump pumps have a built-in fitting for a certain sized discharge pipe, typically 1.25” or 1.5” inside diameter. (Because most sump pump discharge pipe is PVC, determining interior diameter is important. These pipes will be closer to 1.5” and 2” on the outside respectively.)

Using a properly sized pipe for discharge will ensure that the sump pump will perform as the manufacturer intended. Installing discharge pipe of a larger size is not a huge problem and is in fact recommended for longer runs of pipe, typically 20 feet or more.

The real problems occur when the diameter of the discharge pipe is too small. When the discharge pipe is smaller than recommended, the pump’s capacity and efficiency are reduced. The pump cannot push its rated volume of water through the small pipe and the increased friction creates back-pressure that makes it even more difficult to move the water.

End

 

[MikeHalloran]

To tell you the truth, I was working with a mental image of a pneumatically driven diaphragm pump, where a substantial flow restriction would make the 'pocketa-pocketa' noise go more slowly, but if you were counting cycles it wouldn't matter much.

You have included zero information about your actual pump, but now I'm guessing that it's got a large diaphragm, driven through a short stroke indirectly by a piston and a cam or slider-crank.

What puzzles me is why it has a 2" port.

If a 2" port is appropriate for the flow/velocity of the actual discharge, then using smaller accessories in series might result in excessive discharge pressure and perhaps some inaccuracy.

If a 2" port is provided solely for convenience, i.e. because the bung-hole in the supply tank is that big, or to provide structural support for the metering pump, then the size of stuff downstream is not critical.

In either case, if you've got a calibrating cylinder in the system, you may not need to care anyway.

OR maybe the pump has a 2" diaphragm, not a 2" port, which might be more appropriate for a desktop-scale dosing system as illustrated, and we are all shooting in the dark.

Please link to a cut sheet for the actual pump, if possible. Thank you.




Mike Halloran
Pembroke Pines, FL, USA

 

[bmw318be]

Hi,

The types of dosing pump is hydraulic plunger pump.

I link the configuration as followed

The size of 2" are the standard discharge port of the pump.

As we know that size the discharge would result in higher frictional loss, the pump is capable to withstand the higher pressure. Just curious if there is any consideration need to be considered that would be probably more of technical considerations.

 

[bimr]

The Grundfos application guide that you are using is for centrifugal pumps and is incorrect for metering pumps.

"Size piping to accommodate the peak instantaneous flow. Because the reciprocating motion of the pump plunger, pump delivery follows an approximate sine curve with a peak instantaneous flow pi (3.14) time the average flow. Therefore, piping must be designed for a flow 3.14 times the pump capacity; this means that a pump rated for 88 gallons per hour requires piping sufficient for 3.13 x 88 gph, or 276 gph."

 

[Compositepro]

Smaller pipe means high flow velocity, which creates many potential problems with the pulsating flow of reciprocating pumps. The use of a pulsation damper becomes more important.

 

[moltenmetal]

bimr beat me to it: if it's a simplex pump without pulsation dampener, the pipe needs to be sized for pi times the flow, which is an appropriate post for Pi Day. A duplex or reciprocating pump? pi/2 times the flow. A triplex pump out of phase can be sized for the nominal flow.

Pipe from the pulsation dampener downstream can be sized for the nominal flow, but both suction AND discharge up to the pulse dampener need to be sized for pi times the flow.

 

[bmw318be]

Dear Moltenmetal,

Yes it has pulsation dampener which is vertically righ above the pump discharge, after which is back pressure valves.

For simplex pump, do i need to size the pipe to pie times nominal flow ?

I found this link on size vs flow

.

Or from the pump to discharge is crucial to be sized to pie times nominal ?

Any guides how could i determine the pipe size maximum flow such as

3, 2 and 1 inch schedule 80 pvc pipes

 

[bimr]

The site you are looking at is for plumbing and irrigation applications, not industrial applications.

Piping should be sized for simplex pump "pi (3.14) time the average flow".

For industrial applications, the piping should generally be sized for 6-8 ft/sec velocity, unless the piping distance is very short.

3-Inch pipe - 180 gpm

2-Inch pipe - 80 gpm

1-Inch pipe - 20 gpm

Piping is relatively cheap and there is no reason to skimp on pipe. If you have high velocities, then you start having issues such as water hammer, noise, etc.

 

[bmw318be]

Hi does it means

If my flow 1500 LPH, i would need the pipe that could acxomodate 4710 LPH at the discharge line,

If I go with 1 inch 4542 LPH, this would be undersized

The minimum size I believe 1.5 inch ? After the pump ?

My question is after the dampener,correct me if i am wrong that after the dampener the flow would be constant, shall i still need to consider pie x rated flow.


I would like to know if I have externalrelief valve after the pump and before the pulsation dampener, shall i consider the instantenous flow of 3.14 ?

 

[bimr]

That is correct.

Some judgement is necessary. 3 X is probably as appropriate as 3.14 x.

Yes, the pulsation dampener will reduce the surges and a smaller pipe may be used.

 

[bmw318be]

I got it for your logical advise, to be safe it is need to be flowrate x pie 3.14 as we must sized based on worse case if the dampener fail.


Would the pulsation consideration and surge in flow or insantenous flow occur on near the pump let says the vertical straight pipe, if it is a long suction, would the instantenous flow applicable since most of the fluid are near the discharge point end point.

 

[moltenmetal]

Look bmw318be: sizing pipe is an economic optimization between frictional loss (requiring energy from the pump motor to overcome it), and capital cost of piping (piping materials, associated components and labour). That optimum isn't a hard and fast optimum- it varies from situation to situation, fluid to fluid, piping material to piping material etc. There are generic rules of thumb based on velocities, but these are NOT universally optimum under all circumstances. There can be over-riding criteria related to service: slurries and certain corrosive services may have erosional or settling velocity limits which set upper and/or lower bounds on velocity. Or the service might be one where the power of the prime mover is so tiny that you really don't care about frictional loss as long as it's within the limits of the capability of whatever pump you have. It's not something you can just look up from a table unless all you care about is making it "big enough". With plastic pipe, that might be your case, but we have no idea because you haven't told us anything about your service conditions.

For a given system with a given pump putting out a given amount of pressure and a given required flowrate, you can choose to optimize as you wish- as long as, in the end, the frictional loss of your piping doesn't exceed the performance of the pump such that your pump overloads or you do not achieve the flowrate you require. That assumes that the pipe is properly supported and properly mechanically designed of course.

I don't size piping downstream of a pulsation dampener for more than the nominal design flowrate (usually some small prudence margin above the projected operating flowrate). If the pulsation dampener fails, it needs to be replaced or maintained or the capacity loss and pulsation tolerated.

Piping upstream of the pulsation dampener- and note this is almost always the ENTIRE suction line- needs to either be a) sized using the economic pipe diameter selected for pi times the flow, or b) short and simple enough that its frictional loss at pi times the flow is sufficiently low to not affect the desired flowrate.