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Key Considerations for Sizing a Multipurpose Valve for Commercial HVAC Pump Applications  

There are several approaches to sizing an MPV for a Commercial HVAC Pump Application, some that prioritize installation advantages and some that prioritize the reduction of cost. In all cases, the discharge size of the pump, full discharge line size, flow rate, and whether the pump is constant or variable speed should be considered. 


An MPV, or multipurpose valve (Fig, 1), combines all the functions that are normally required on the discharge side of a centrifugal pump in a hydronic heating or cooling system into one body. It is a Shut-Off Valve, a Non-Slam Check Valve, a Balancing Valve, and a Flow Metering Valve. The Taco Plus Two MPV, one of many solutions offered by Hoffman Hydronics, is uniquely field convertible from a straight pattern to a 90-degree right angle pattern (Fig. 2) providing the additional and optional function of an elbow. 


Figure 1


Figure 2



Utilizing an MPV instead of separate valves on a pump discharge reduces the number of components requiring installation and lowers labor costs. 

Optimum performance is achieved by keeping the maximum pressure drop under 4 PSI. Data generated by manufacturers provides the pressure drop by GPM for a given MPV model/size (Fig. 3). 


Figure 3

For a pump in a constant-speed application, it is generally advised to select an MPV between the size of the pump discharge and the full line size of the discharge pipe. To utilize the Balancing Valve function of the MPV for system balancing, a minimum pressure drop of 1 PSI is recommended for accurate flow rate measurement. Pressure drop can be calculated from Eq.1., where GPM is the flowrate, Cv is the value coefficient supplied by the manufacturer, and delta P is the pressure drop. 


GPM = Cv x √ Δ P (Eq. 1) 


Calculations may indicate that a reduction of discharge size is necessary to achieve the 1 PSI drop recommended; however, the provided Cv is based on an MPV at the 100% open position. It is possible to throttle the valve in the field to achieve the necessary pressure drop for an accurate flow rate reading.  


This adjustment prevents installers from having to reduce from the discharge of a pump to an MPV only to increase after the valve to the discharge pipe size. In conclusion, sizing an MPV between the pump discharge size and the discharge pipe size for installation convenience will suffice so long as the functionality of the MPV is understood and a 4 PSI is not exceeded.  


If a pump is controlled by a VFD (variable frequency drive), the MPV can be sized based on the discharge pipe size to provide the easiest installation option in the field. Some installers may prefer saving cost on the valve by selecting the smallest MPV that does not exceed 4 PSI of pressure drop and increasing after the valve to the full discharge pipe size. 


The Taco Plus Two MPV recommends a 12” spool piece for applications of 6” flange size or less between the pump discharge and the entrance to the MPV (24” spool piece for applications greater than 6”) to reduce noise (check valve chatter) and ensure the proper flow condition for metering. Hoffman Hydronics, through Metraflex, offers a pump discharge accessory that serves as a flow conditioner and increaser all in one (Fig. 4), the Metraflex Vaneflex. 

 

Figure 4

Sizing an MPV for commercial HVAC pump applications requires careful consideration of several factors, including flow rate, pressure rating, pump compatibility, and piping configuration. By selecting the right MPV and ensuring proper installation, you can optimize the performance and efficiency of your HVAC system. 


The sales representatives at Hoffman Hydronics can answer any of your questions about the MPV selection process. Contact your sales representative today with questions about MPVs and other accessories that can save you time and money in the field!



 

 

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