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Belimo Pressure Independent Control Valve Design Considerations:

 

The PICCV valve requires 5 psi differential pressure to provide stable operation.  Belimo asserts that a control valve selected at 3 psi, and a balancing valve at 2 psi result in an equal pressure requirement.  If the balance valve is omitted, the pressure dependent valve requires less pressure to operate.

 

There are three criteria to consider when selecting PICCV valves:

 

1.                         The brass valve body maximum flow rate (denoted by the orange horizontal lines in the catalog).

2. The programmed flow rate of the actuator.

3. The piping package with unions should be selected with all valve installations, this guarantees the correct number and locations of test ports to properly assess valve operation.

 

While the programmed flow rate of the actuator can be modified up to the brass valve body peak flow rate utilizing either the electronic "tool" or a cable and software, the valve body maximum flow rate is set, and cannot be altered.

 

The actuator flow rate, set from the factory allows the valve only to modulate over the preset maximum flow range.  Therefore, a 10V signal from the control system will not cause the valve to open 100%, it will open to the designated position that results in the maximum program flow.

 

 

Current valve selection process:

 

Pressure independent control valves are currently selected either with the aid of the Belimo valve selection software, or utilizing the printed catalog distributed by the local representative.  The valves are selected by flow rate, rounded up to the nearest 0.5 gpm.

 

As shown below, this method can result in several unintended consequences:

 

       Undersized Valves:  The energy conservation code requires that the heating airflow reduce to 30% of the peak cooling airflow.  This forces an otherwise conservative engineer to use the bare minimum airflow required for heating, regardless of diffuser location, throw, and room orientation.   In retrofit applications, this has the potential to undersize the heating load, and therefore the hot water flow rate and valve size.

       Water Stagnation:  The specification and installation of valves below 1 gpm has the potential to cause stagnation and insufficient flow rates to flush debris from piping systems.  Where a normal valve can be opened to 100% to allow the system to be flushed, the pre-programmed valve will not, causing insufficient flow to clear the piping of debris.

       Increased Maintenance:  Water stagnation, and poor water quality lead to increased strainer, valve and water maintenance time.

       Decreased Occupant Comfort:  The design engineer designs to a peak load, often exceeded, if only for a few hours per year.  However, this situation, combined with stagnation and poor heat transfer due to buildup in the piping system may lead to unacceptable space comfort.

       Increased Commissioning Cost:  Adjusting the valve flow rate after the fact requires a technician to access the valve, disconnect  the BAS connection, plug in the tool, recalibrate the valve, and reconnect the BAS system.  Based on the sheer number of terminal units in a VAV system, the cost of this time is not warranted.

 

The chart show below shows the allowable flow error as designed, and the allowable flow error to be adjusted before assembly replacement.

 

 

The allowable error in the max heating airflow is unachievable with the current design and construction tools, and is certainly so in retrofit applications.

 

While some of the selections allow for a reasonable amount of error in load calculation and building construction with the maximum adjustment of the valve body, some of the selections do not allow any error.  All of these installations result in an unacceptable level of avoidable maintenance time to correct even the slightest miscalculation.

 


Recommendations:

 

An alternate method of selection would not select specific valve model numbers down to 1/2 gpm, but would select them based only on the maximum flow rate of the valve assemblies, with an allowance for flow rates near that maximum.  This method has been confirmed with the local Belimo representative, and is currently applied in their European market. The chart below shows an example selection.

 

 

This selection criteria allows for the following operational and maintenance procedures:

 

       Valve flow limits, which are based on a software input, are now controlled at the BAS level, not the device level.  This allows for fine tuning and corrective action from the control system, not above the ceiling.

       During part load or lower-than-predict load, the system functions the same as the current selection method due to the proportioned characteristics of the valve assembly.

       Increased space comfort, even on the days outside the ASHRAE 99% range.

       The airflow adjustment allowances should result in fewer errors requiring field modification or equipment replacement.

       Uniform valve selection from in-house design and controls, to maintenance replacement and consultant engineer projects.

       Reduced commissioning time.

       Allows for the implementation of a water circulation routine to maintain water quality throughout the system during all seasons.  Increased water quality results in greater heat transfer, lower energy use, and decrease system maintenance.

       A water circulation routine would lock all control valves to 100%, and circulate the entire pumping system for 15 minutes at a frequency to be determined.  This would help alleviate buildup and stagnation, particularly in the low flow or "dead leg" portions of the hydronic system.

 

The chart below can be used to input a output signal limit for limiting flow per the design documents.