14 replies to this topic

[Chemical.Engineer]

Hello

1- I want to select pumps for potable water system in power plant. the average consumption is 6 m3/h and daily maximum consumption is 288 m3/day in a year. what is the best selection for this system due to fluctuations in consumption from zero to more than 12 m3/h in a day?

 

2- If a pump with the design flow rate of 12 m3/h is used and for example the consumption falls to less than 2 m3/h for several hours in a day, what happens to the pump and consumers?

And what measures should be considered in this case(pump with design flow rate of 12 and consumption flow in afew hours less than 2 m3/h) for pump and piping?

Edited by Chemical.Engineer, 07 October 2015 - 07:45 AM.

[samayaraj]

Hi,

 

Its better to go for a overhead tank connected with a pump. Once the tank attains low level, pump should start and the same shall be stopped at high level.

Edited by samayaraj, 07 October 2015 - 10:35 AM.

[ankur2061]

Hi,

 

Its better to go for a overhead tank connected with a pump. Once the tank attains low level, pump should start and the same shall be stopped at high level.

Samayraj,

 

This is for protection of pump and not for fluctuating demand.

 

Chemical.Engineer,

 

Assuming that your pump is a centrifugal pump, you should always have a minimum flow recycle from the pump discharge. When your demand (forward flow) reduces, the pump will recycle the flow to the suction side of the pump (preferably the suction tank). If there is a restriction orifice or control valve in the recycle line sized for the minimum safe continuous flow the pump will be in operation all the time despite zero demand (forward flow). This means more energy costs due to continuous operation of pump. If energy costs are not an issue this scheme will work fine without any problems for the pump.

 

If you are really looking to operate the pump intermittently, only when there is demand (forward flow), and want to save energy the best option would be to provide a "dual hi-low pressure switch" on the discharge side of the pump. On high discharge pressure (no demand or no forward flow), the pump will trip and on low discharge pressure (demand or forward flow) the pump will start.

 

Hope this helps.

 

Regards,

Ankur.

[fallah]

Chemical Engineer,

 

Potable water distribution pumps in an industrial plant such as a power plant will normally be designed so that to deliver continuously the potable water at design flow rate (here 12 m3/hr) and at operating pressure.

In normal operation, part (or whole in case of no demand) of the potable water flow rate will be recirculated to the suction tank through a heat exchanger to cool down the potable water, if needed. Low low flow on distribution pumps discharge line will start spare pump.

[Chemical.Engineer]

Dear ankur2061 and fallah

Thank you. I have considered a juckey pump with design flowrate of 12 m3/h and 2*100 pumps with design flowrate of 12 m3/h which in low pressure will start. I have 2 questions:

1- Given that the average consumption is 6 m3/h, is right to select a pump with 12 m3/h for juckey pump?With regards to standards and criteria do not recommend you the pump with lower design flow?

2- What size should be selected for minimum flow line size?The minimum flow line size is usually 30% of the design flow rate of pump. Due to the possible very low consumption close to zero in some time, this means that should be consdered minimum flow line size about 100% of the design flow rate of pump?is this common in industry and is used?

[fallah]

Chemical Engineer,

 

Would you please upload a simple sketch of the system you described....

[Chemical.Engineer]

fallah,

I have attached a simple sketch. If more detail is required i can upload a more complete figure.

Attached Files

•  sketch.docx   28.89KB   31 downloads

Edited by Chemical.Engineer, 08 October 2015 - 12:10 PM.

[fallah]

Chemical Engineer,

 

Appears you are trying to follow pumping system which is normally applied in fire fighting systems...if so, please clarify about the reason(s)...

[Chemical.Engineer]

fallah,

This arrangement is common in the industry. A continuously operated jockey pump is used to supply potable water to the distribution header and maintains the pressure of the system during low-flow requirement periods:

 

http://www.nrc.gov/r...r 9/9-2_r11.pdf

 

http://wolfeconsulti...potable-water/ 

(last paragraph)

[fallah]

Chemical Engineer,

 

Yes, the potable water network could be arranged with a jockey pump (to maintain network pressure) and main pumps like fire water pumping system; but i think it's not so common arrangement for potable water in industry

[Chemical.Engineer]

Chemical Engineer,

 

Yes, the potable water network could be arranged with a jockey pump (to maintain network pressure) and main pumps like fire water pumping system; but i think it's not so common arrangement for potable water in industry

what is your desirde arrangement for this system?(number of pumps, design flowrate of each pump and minimum flow line size- system specification: average consumption of 6 m3/h and maximum daily consumption of 288 m3/day)

[fallah]

 

what is your desirde arrangement for this system?(number of pumps, design flowrate of each pump and minimum flow line size- system specification: average consumption of 6 m3/h and maximum daily consumption of 288 m3/day)

 

 

Chemical Engineer,

 

I think the desired arrangement is so that part of design flow rate, say normally around %25, being recirculated and cooled through a heat exchanger to maintain stored potable water below the temperature of almost 30C and also to prevent biological growth in the network.

Then if the design flow would be 12 m3/hr, you need two recirculating pumps (1 Working+1 Stand By) with the capacity of each pump equal to 15 (12 design flow+3 recirculating flow which you titled it as minimum flow) m3/hr...and an exchanger in recirculating line if ambient temperature is above 25-30C...

[curious_cat]

 

 

what is your desirde arrangement for this system?(number of pumps, design flowrate of each pump and minimum flow line size- system specification: average consumption of 6 m3/h and maximum daily consumption of 288 m3/day)

 

 

I think the desired arrangement is so that part of design flow rate, say normally around %25, being recirculated and cooled through a heat exchanger to maintain stored potable water below the temperature of almost 30C and also to prevent biological growth in the network.

 

 

What do you use as a cooling fluid in this HEX to keep the water below 30 C? Chilled water?

[fallah]

 

What do you use as a cooling fluid in this HEX to keep the water below 30 C? Chilled water?

 

 

curious_cat,

 

Cooling water from cooling water package would do the job...
 

[curious_cat]

 

 

What do you use as a cooling fluid in this HEX to keep the water below 30 C? Chilled water?

 

 

curious_cat,

 

Cooling water from cooling water package would do the job...

 

Thanks Fallah!

 

I wondered because our plant's CW is typically around 29 C. That would give very little deltaT to give 30 C exit temperature.