Pressure-independent control valves

Honeywell has released a new range of Kombi-QM pressure-independent control valves (PICVs) suitable for a wide range of hydronic applications in the building services industry.

Fan-coil units and chilled beams are probably the most familiar applications of PICVs with the move from 4-port to 2-port valves driven primarily by the need to reduce excessive energy consumption of pumps and thermal losses through pipework.

In the selection of 2-port valves for use in variable-flow systems, particular attention is given to some of the issues that can arise in systems where pump speed is designed to change in response to thermal demand.

Fluctuations of flow initiated by the positioning of the 2-port valves in response to varying occupancy levels and heat losses causes pressure changes in the system, resulting in instability of flow through all the valves.

The system is effectively unbalanced, resulting in the valves ‘hunting’ as they constantly try to maintain control.

An unstable system has a direct impact on energy consumption, occupancy comfort, noise and maintenance costs.

To ensure accurate temperature control in the occupied spaces of buildings where the system pressure is maintained by a variable- or constant-speed pump installation, it is crucial that pressure fluctuations do not affect the flow through terminal units.

The solution is to install 2-port control valves that can maintain close control of flow independently of the system pressure variation caused by changes in pump speed or the operation of other valves.

Pressure-independent control valves are exactly what the name suggests. They maintain a constant pre-set differential pressure across a control valve such that control action of the valve is not affected by inlet-pressure instability.

A PICV incorporates three critical functions.

Pressure regulation: A spring-operated diaphragm valve at the inlet of the PICV automatically adjusts the differential pressure across the inlet and outlet ports to maintain a constant value pre-set by adjusting the spring tension.

This ensures (providing the range of inlet pressure variations are within the valves specification) that the differential pressure across the complete valve will remain constant within its specified tolerances.

Effectively, this ensures that the flow setting and control of the valve will not be affected by varying inlet pressures; i.e. it is independent of inlet-pressure changes.

Flow Regulation: An adjustable orifice allows the flow through a PICV to be adjusted to the designed flow rate. The orifice is at the outlet of the PICV and, combined with the function of the pressure-regulating valve, ensures that the design flow rate is maintained irrespective of varying inlet pressures.

Once the flow regulator has been pre-set to the desired flowrate and the pressure regulator adjusted to the required differential pressure, a constant pre-set flow will be maintained.

A valve that has the combination of pressure and flow regulation is an effective device for maintaining a constant flow rate through downstream pipework. These are essentially pressure-independent constant-flow valves without the valve control function and actuator. The addition of actuator and valve provides the control element to the PICV

Flow Control: The control function of a PICVis a remotely actuated valve located between the pressure and flow regulators.

Opening and closing the control valve varies the flow through the PICV, providing the control function that will respond to an input signal from a discrete terminal controller or a BMS.

The maximum flow is set by the flow regulator, and the required differential pressure is maintained by the pressure regulator — enabling the control valve to give accurate control independent of fluctuations of inlet or line pressure.

The above describes the function of a PICV in terms of three separate valves. However, for practical considerations and efficiency of design the functions of the flow regulation and control are often combined. The position of the control valve seat would be adjusted in relation to the valve plug to set the maximum flow and would effectively be the flow regulator. Control is then effected by the actuator moving the valve plug incrementally between the pre-set fully open and closed position.

The relationship between control input signal and the movement of the actuated valve stem and the flow defines the valve’s control accuracy.

Where the corresponding percentage movement of the valve is equal to the percentage flow, the control characteristic is called equal percentage.

Equal percentage valves provide the best solution for systems where the heating and cooling have forced-air fans, such as fan-coil units.

Accurate set up and commissioning is essential for PICVs. Self-sealing test connections are usually integrated into the valve body.

Test point positions are located at the inlet to the pressure regulator and the outlet of the flow regulator or combined control/flow regulator to ensure that the differential pressure is taken across the complete valve.

The flow regulator is marked with precise graduations, providing accurate adjustment of the valve orifice to pre-set the designed maximum flow.

The new range of Honeywell Kombi-QM PICVs is suitable for pipe sizes DN15 to DN150 and categorised in four groups.

5 Group 1: DN15 to DN25

5 Group 2: DN20 to DN32

5 Group 3: DN40 to DN65

5 Group 4:DN50 to DN150

This extensive range covers the full spectrum of applications for PICVs — from fan-coil units and chilled ceilings to air-handling units and single-pipe heating systems. Groups 1 and 2 utilise linear actuators. Groups 2 and 3 have rotary actuators.

PICVs are now specified in the majority of fan coil projects, with the BMS controlling the pump speed and differential pressure.

Terminal-unit applications are eminently suitable for Groups 1 and 2 of the Honeywell Kombi-QM range and will be the main focus of this article. A further article on the operation and application of the larger PICVs (Groups 3 and 4) will be in a future Bulletin.

Kombi-QM valves are available in pipe sizes DN 15, 20, 25 and 32 — giving a high degree of flexibility to design engineers.

A full range of linear actuators is available to provide on/off, 3-point and 0 to 10 V modulating control . A non-electric actuator is available in Groups 1 and 2.

The Kombi-QM pressure independent control valve combines a fully stroked, full-authority flow controller with pressure and flow regulation adjustable with an actuator fitted.

Installed without an actuator, the Kombi-QM valve can be used as a constant flow limiter, the flow being set by a visual graduated scale that can be adjusted without the need of hand tools.

The integrated pressure-test points and accurate flow-measuring dial provide convenient access on one side of the valve body. Commissioning of the valve and ascertaining optimal pump setting can be achieved without the need for complex calculations.

With the system running, flow pre-setting and adjustments can be made with the actuator in position and operational. The valve is maintenance friendly and has a plastic cap for temporary emergency shut off.

The Kombi-QM is installed in the return pipework and suitable for water or water/glycol (VDI 2035) from -20 to 120°C.

Maximum pressure rating of Groups 1 and 2 of the Kombi-QM range is 25 bar with maximum differential pressure of 400 kPa.

Significant set-up and commissioning time is saved by the ability to adjust the flow regulation without removing the actuator. With the actuator driven to the fully open position, the maximum design flow in litres/hour for the PICV can be set by adjusting the graduated scale of the flow regulator. The differential pressure in kPa across the Kombi-QM is set using the SafeCon connectors connected to a portable manometer.

Kombi-QM PICVs have internal threads with the facility of fitting faced connections for easy installation and replacement.

PICVs are essential in providing the stable final control flow element of the terminal units. However, the efficiency of the complete system is dependent on the BMS. An effective BMS strategy for pump speed is critical in matching the heating and cooling demand controlled by terminal field controllers in occupied areas.

Pump speed is referenced by locating differential-pressure sensors near the furthest PICV from the pump installation or at various locations across the flow and return. It is most important to provide accurate representation of the system pressure to enable the BMS to control the pump speed to maintain the designed system differential pressure.

Jim Sword, UK sales leader for CentraLine by Honeywell and Ex-Or, comments, ‘The release of the Honeywell Kombi-QM PICVs is a welcome addition strengthening our CentraLine BMS portfolio.

‘We now have one of the most comprehensive range of building-control products for the commercial sector — from the Niagara-based Arena AX supervisor and the Hawk integration controller through to the Spyder BACnet and LON terminal controllers for a multiple of applications, including fan-coil units, variable-air-volume and chilled-beam ceilings.

‘PICVs are now almost an industry standard for the efficient and effective control of occupied spaces in buildings with terminal units using 2- port valves.

‘Combined with our wide range of building-management products and systems we are confident we can offer our Partners and their clients a seamless integrated building-management solution — from the supervisor to the individual control valves.

My team and I are looking forward to introducing this new range of PICVs to our CentraLine Partner network and discussing how we can support them with future projects.’

If you would like to know more about CentraLine and the Kombi-QM product range, see the link below.

For more information on this story, click here: November 2015, 21
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