Changing pressures

Frese, Commissioning, PICV, DPCV, PIBCV, balancing
Fig. 1: Some designs of pressure-independent control valve allow for differential pressure to be measured across the regulated orifice (P1 to P2 in this diagram)

Stephen Hart looks at the technology that is available to maintain the balance of hydronic systems when the pump speed is reduced to save energy.

Most medium to large commercial hydronic heating and cooling systems designed and installed today include inverter-driven circulating pumps and 2-port automatic temperature-control valves so that the pumps can reduce speed and energy consumption during periods of low heating or cooling demand. These variable-flow pipework systems must be designed with self-balancing pipework or self-acting regulating valves to accommodate pressure fluctuations caused by pumps changing speed or control valves throttling.

Flow-limiting (FLV), differential-pressure-control (DPCV) and pressure-independent balancing and control (PIBCV) valves are all self-acting and close a variable orifice against a spring to maintain a stable flow rate through the valve — independent of any pressure fluctuations in the pipework. The flow is then regulated manually or via an actuator. The ability for the valve to maintain a stable flow across the controlled pressure range will be determined by the valve design and proportional band of the internal spring. A measured pressure drop across the valve (P1 to P3 in Fig. 1) will establish whether the valve is operating within its controlled pressure range and enable calculation of the flow rate if the spring has not lifted from its seat, a point known as the ‘start-up’ pressure.

Some designs of pressure-independent designs allow for differential pressure across the regulated orifice to be measured (P1 to P2 in Fig, 1) which, when combined with the stem position and manufacturer’s data, can be used to calculate the flow through the valve. However, as with a variable-orifice double-regulating valve (VODRV) it is unlikely that the manufacturing repeatability and the resolution of the flow setting mechanism is accurate enough to meet the tolerance limits required for flow regulation accepted under CIBSE Code W. For this reason witnessing authorities generally require a separate method of flow measurement, typically a fixed-orifice flow-measurement device (FOFMD) such as a metering station or an orifice plate.

Fig. 2: Frese PVS differential pressure and flow control valve.

Care should be taken to select a DPCV according to the design flow rate through the valve and the expected pressure loss in the downstream branch at the design flow rate. During commissioning DPCVs should be adjusted until the flow rate measured through an adjacent FOFMD is equal to the design flow within the specified tolerance limits, and the actual pressure drop in the downstream branch should be recorded. The actual flow in the branch will be affected by any changes in position of the downstream balancing or control valves and the effect of pressure fluctuations on the spring inside the DPCV. For this reason a combined DPCV and FLV has proven popular (Fig. 2) at landlord and tenant interfaces to ensure that flow in the landlord’s upstream pipework is not affected by any subsequent adjustment of the downstream pipework or regulating valves by the tenant.

FLV’s and PIBCVs are generally installed in terminal unit or plant equipment branches and are selected in accordance with line size and dependent upon the design flow rate. CIBSE Code W allows the designer to decide if it is necessary for flow-measurement devices to be installed on the same branch as PICVs. It may be acceptable to the witnessing authority to record the measured flow rate in a branch containing multiple terminal units in addition to the pressure loss across the critical valve to prove it is controlling flow as expected. In any case the expectation of the witnessing authority, system designer and commissioning-management organisation should agree flow-verification requirements early in the project to ensure FOFMDs are installed where required.

Unlike manual balancing valves FLVs and PIBCVs do not require a FOFMD for the flow to be set during the commissioning stage. Instead the flow is set in accordance with a graph, table or scale provided by the valve manufacturer. The FOFMDs are for flow-verification purposes only. However, test points are commonly supplied on FLVs and PIBCVs to confirm valves are operating within their controlled pressure range and, in the case of the critical valve on each circuit or sub-circuit, to allow the upstream pump or DPCV to be set for maximum system efficiency. In the case of the pump, the speed can be reduced until the measured differential pressure across the critical valve is just above its start-up pressure so that the required pump head can be minimised. (Fig. 3).

Fig. 3: For maximum system efficiency the speed of the pump can be reduced until the differential pressure across the critical valve (highlighted) is just above its start-up pressure.

All self-acting valves render upstream regulating valves redundant unless the maximum head generated by the pump is less than the operating pressure control range of the self-acting valve. Larger DRVs should be removed and replaced by less-expensive isolation valves and FMDs to save capital cost and remove the temptation to throttle upstream balancing valves during commissioning, which would add to system resistance and cause self-balancing valves to drop out of their controlled pressure range.

Stephen Hart is managing director of Frese.

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