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Potable Water Distribution Design for communities - use of PRV's

Hi all,

I have a general question on the use of PRV's at individual services for a "closed" distribution system, i.e. pumps directly draw from a treated reservoir and feed to end-users with no elevated/pressure tank and storage elsewhere.

I have come across an existing design for a partially built-out community that has its own treatment system. Most of the pumps are on VFD's and have control logics programmed to act according to the pressure indicating transmitter to "control" the output pressure as needed. I did not see a main PRV downstream of the pump headers and in the grid.

Is it a good practice to have PRV's installed on all the housing services, regardless of the pressures indicated by simulations under min flow, ADD, MDD and etc.? My thought was that surges can occur, and if control is not operating as it should, pumps may deliver at full speed all the way to the shutoff head when demands are low. I'm also not sure if rupture discs and such were installed at the pump station for protection (I may find out).

Thanks in advance for your insights!

Anthony

Parents
  • Hello Anthony, 

    Generally PRVs are installed at DMA / pressure zone boundaries and also to reduce pressures to defined values in DMAs. In small systems without storage, if the pumps are maintaining the required pressure in the system, then PRV will not be installed unless required. Its not a best practice though to install PRVs on all housing services. 

    How does the Pressure Reducing Valve (PRV) work in WaterGEMS and WaterCAD?

    We haven't heard of PRVs being used specifically for surge protection before. They should work well with reducing "static" (steady state) pressure, by automatically throttling. During the transient simulation, they are essentially a fixed discharge coefficient. During the transient simulation, HAMMER uses a single discharge coefficient for the PRV, based on the headloss from the initial conditions (necessary to reduce the pressure per the PRV setting.) Headloss through the PRV during the transient simulation varies with flow according to that discharge coefficient. This should dampen surge waves similar to any other head loss, such as from pipe friction.

    If you are concerned about surge protection of distribution system in case controls are not operating as expected, then you can perform surge analysis in Hammer by using the same model created in WaterGEMS and check for surges, then if required you can suggest surge protection devices e.g. surge tank. 

    If I haven't understood your question, then please elaborate your query. 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

Reply
  • Hello Anthony, 

    Generally PRVs are installed at DMA / pressure zone boundaries and also to reduce pressures to defined values in DMAs. In small systems without storage, if the pumps are maintaining the required pressure in the system, then PRV will not be installed unless required. Its not a best practice though to install PRVs on all housing services. 

    How does the Pressure Reducing Valve (PRV) work in WaterGEMS and WaterCAD?

    We haven't heard of PRVs being used specifically for surge protection before. They should work well with reducing "static" (steady state) pressure, by automatically throttling. During the transient simulation, they are essentially a fixed discharge coefficient. During the transient simulation, HAMMER uses a single discharge coefficient for the PRV, based on the headloss from the initial conditions (necessary to reduce the pressure per the PRV setting.) Headloss through the PRV during the transient simulation varies with flow according to that discharge coefficient. This should dampen surge waves similar to any other head loss, such as from pipe friction.

    If you are concerned about surge protection of distribution system in case controls are not operating as expected, then you can perform surge analysis in Hammer by using the same model created in WaterGEMS and check for surges, then if required you can suggest surge protection devices e.g. surge tank. 

    If I haven't understood your question, then please elaborate your query. 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

Children
  • Thanks Sushma. I guess I didn't make it clear - what I meant by PRV was actually the small pressure relief valves that one would install in a dwelling unit to protect fixtures from the higher pressures coming from the municipal system. More of a plumbing device (https://actonwater.com/customer-service/pressure-reducer).

    My concern was more on the excess pressures pumps may introduce to the system rather than surges (I mixed them up at initial query). The real-life scenario is when the demand is low, pump(s) start to operate towards the left of the pump curves, the increasing head is captured by equipment which then tell the VFD to lower speeds or shut off. These controls more or less keep the pump head at a maximum pressure threshold, but what if it fails to detect the rising pressure and pumps continue to operate at full speed? Because there are no storage tanks to fill, water practically has nowhere to go and pressure will continue to increase in the system until it reaches the pump's shutoff head. 

    I wasn't after those pressure reducing valves installed on main lines that define pressure zone boundaries. This system in particular is small with relatively flat topography so there was no need to have more than one pressure zones.

    I understood your explanation on using transient simulation in Hammer to determine the need for surge tanks.

    Apologies if above is outside the modelling aspects we typically discuss here.

  • In your system, it sounds as if the PRVs are just a backup to the VFDs to control pressure. They are just insurance and are may be worthwhile. What are the typical pressure in the system and what are the PRVs set to maintain downstream.

    If you keep the pressure in the street down to 80-100 psi, you probably don't need to house PRVs. If it is possible to get the pressure up to 150 psi or so, then you had best have the PRVs.

    Sometimes pumps are oversized and waste a lot of energy. Depending on how much you need to turn down the VFD (which wastes energy), it may pay to trim or replace some impellers unless the high flow is needed at times. Then you wouldn't need to PRVs to waste energy.