FAQ - Pressure Independent Control Valve - PICV

Q: What is a PICV valve and where can it be used?

Pressure Independent Control Valves (PICV) 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. PICVs are suitable for a wide range of hydronic applications in the building services industry. Fan-coil units, air handling units and chilled beams are probably the most familiar applications of pressure independent control valves with the move from 3-ports to 2-ports valves driven primarily by the need to reduce excessive energy consumption of pumps and thermal losses through pipework. This group of products is called LIBRA in the iSMA CONTROLLI Solutions.

Q: What are the benefits of using a PICV?

LIBRA brings benefits like:

• LIBRA combines 3 valves in 1 with a relevant installation and commissioning cost saving;

• LIBRA commissioning is extremely easy due to the possibility to set the design flow on each valve by means of the manual knob

• LIBRA valve has this adjustment knob on the bottom of the valve making this operation extremely easy without the need of removing the actuator

• Furthermore LIBRA design flow regulation does not reduce the control stroke offering a 100% authority in all conditions

• LIBRA allows a dynamic hydraulic balancing and therefore commissioning does not have to be repeated in case of changes in the system

• Max. flow regulation does not reduce the control stroke: 100% stroke always

Q: How can I size a PICV?

Sizing a PICV correctly ensures optimal system performance, energy efficiency, and comfort levels. To correctly calculate PICV valve size you need:

• design flow

• pressure drop

• control type

• pipe size

Q: How can I calculate flow rate from a PICV?

The flow rate cannot be directly measured from the PICV. An alternative method of measurement that we can offer is to install the "venturi pipe" accessory as part of the system and using the chart below.

Q: What are the measuring points used for?

Measuring points on a Pressure Independent Control Valve (PICV) are crucial for monitoring and verifying the performance of the valve and the overall HVAC system. These points allow for the measurement of pressure, and differential pressure across the valve. Understanding and utilizing these measuring points ensures that the system operates efficiently and that the PICV is set correctly.

Q: What is the start-up differential pressure and what does it depend on?

The start-up differential pressure is the pressure difference required across a PICV for it to operate correctly and maintain the set flow rate. This differential pressure ensures that the valve can adjust itself to provide a constant flow, independent of fluctuations in system pressure. It has to be noticed that the start-up differential pressure depends on the max flow setting: the lower is the max flow the lower is the start-up differential pressure.

Q: What is the hysteresis and does it affect the system performance?

The accuracy with which the flow rate setting is maintained also depends on whether the pressure differential across the valve is rising or falling. It can be seen from the following figure that there are distinct rising and falling pressure curves. The difference between the two curves is often referred to as the valve’s “hysteresis”. The hysteresis effect is caused by the sealing elements in the pressure regulating part of the valve. Due to hysteresis, two repeatable flow readings can be obtained depending on whether the pressure differential across the valve has risen or fallen to the value when the measurement is taken. Since the valves are factory tested on their rising pressure curves, the flow setting device indicates flows that correspond to a rising rather than decreasing pressure differential. For the reasons explained, the valve’s proportional band and hysteresis may cause flow values to vary from their set values.

Q: What is the Differential Pressure Control and how does it work?

Differential Pressure Control involves maintaining a consistent pressure difference between two points in a hydraulic system. This pressure difference is crucial for ensuring the proper operation of PICV control valves and other devices that regulate fluid flow in HVAC systems, heating systems, and cooling system. 

A spring-operated diaphragm valve at the inlet of the valve automatically adjusts the differential pressure across the inlet and outlet ports to maintain a constant. One side of the diaphragm is in contact with water from the inlet to the valve at a pressure P1 whereas the other side is in contact with water from the outlet to the valve at a pressure P4. This means that if there is any change in the differential pressure P1 to P4, the position of the differential pressure regulator will also change. The result will be that the differential pressure P3 to P4 (i.e. from downstream of the differential pressure regulator to the valve outlet) will remain constant at all times regardless of changes in the overall differential pressure P1 to P4. This ensures (providing the range of inlet pressure variations are within the valves specification) that the differential pressure across the flow control valve will remain constant within its specified tolerances.

Q: What is the valve authority and how can I calculate it?

Valve authority concept is used to express in a simple way a control valves’s share of the pressure drop in relation to the total pressure drop of the controlled circuit. It is mathematically defined as follows:

∆Pvalve is the pressure drop of the valve at the design flow and 100% opening

∆Pcoil is the pressure drop of the coil on which the valve is changing the flow

∆Pother is the pressure drop of the other components of the controlled circuit

In other words the valve authority express the capability of the valve to control the flow according to the flow characteristics when installed in a system; the lower is the authority the higher is the distortion of the valve characteristics and therefore the lower is the capability of the valve to control at part loads.  A very good equal percentage valve, but badly sized with a low authority, will provide a linear or quick opening control characteristic.

Q: What is a PICV presetting and how do I set the correct flow?

We set the presetting using the adjustment knob. The PICV setting knob is a device with a numbered scale that is responsible for adjusting the maximum flow rate of the PICV valve (Open 100%) to the requested design flow rate. 

Due to the fact that PICV is equipped with a maximum flow regulator, the characteristics of curves depends on the current position of the flow adjustment knob.

Q: What is the flow characteristic of a PICV?

PICV can be characterized by two different flow curves:

• flow vs valve opening;

• flow vs differential pressure.

The first curve (figure 1) shows how the flow changes according to the valve opening from fully closed to fully open condition.

The second curve (figure 2) shows that PICV has a minimum and maximum pressure differential value below or above which the valve will not compensate differential pressure changes. If the pressure differential is less than the minimum value, the spring inside the pressure regulator remains fully extended, whereas at pressure differentials greater than the maximum value, the spring is fully compressed. Under both of these conditions the pressure control element in the valve acts as a fixed resistance; the valve can only control flow when the spring is under some degree of partial compression. The “operating range” of the valve is the range of differential pressures for which control is possible.

Q: What certifications does PICV have?

PICV valves are regulated by the Pressure Equipment Directive of the European Union. In special cases, we also provide declarations such as:

• Internal testing declaration from R&D on special request from customer (endurance test or repositioning test or stability test or different test)

• Production test declaration prepared from Production for valves whit list of production test according to standards (like leakage, pressure test etc…)

Q: How to mount a PICV correctly?

PICV can be mounted in either the flow or return pipework serving terminal units. Can be mounted with any orientation but not up-side-down. Consideration should be given to the flushing regime when deciding on the position of the valve in order to minimize the risk that large contaminants circulating in the system can damage the valve.

Q: Are a PICV suitable for dirty water?

PICV can be sensitive to high levels of particulate dirt which causes fouling of the low pressure areas within the valve. Strainers are not effective enough at removing this kind dirt from the media as the mesh size usually installed is too large to trap such tiny particles. This kind of fouling can only be prevented by ensuring a water quality of a high standard by on-going water treatment and filtration.  We recommend the water or water/glycol mixture must always be clean, free of debris and treated in accordance with VDI2035.

Q: What is the range of diameter and flow rate for a PICV in iC Solution?

iSMA CONTROLLI is able to supply PICV valves according to the following specification:

Q: How to select PICV size and compatible actuator?

For an easy and fast selection of the Libra PICV, a MS Excel software tool is available.

The user has to fill in a table with the quantities and the design flow rate of the requested PICV, select power supply, control type of the requested actuator and the software will automatically produce a bill of material.

Q: What I should to do before first commissioning?

The commissioning procedure is as follows:

• For the selected VLX valve, ensure that the 2 port valve is fully open. Measure the pressure differential across its pressure tappings and confirm that the value obtained is greater than the minimum value indicated in the product brochure. If this is not the case investigate the causes and, if necessary, report to the designer.

• Position the max flow adjustment knob to the specified design flow rate (for VLX5, VLX5P, VLX6P and VLX8P model use the locking screw to fix the position) and record the setting.

• Repeat the above process for all of the LIBRA valves on the branch.

• Measure the flow rate indicated at the flow measurement device on the branch. Confirm that the value recorded is equal to the sum of the flows set at downstream LIBRA valves. If this is not the case investigate the causes and, if necessary, report to the designer.

• Repeat this procedure until all LIBRA valves in the system have been set and their summated flows checked against upstream flow measurement devices.

• Measure the differential pressure across the LIBRA on the system index terminal (usually the most remote terminal from the pump. Adjust the pump speed until the pressure differential across valve is equal to the minimum value indicated in the product brochure. Please consider that if the valve on the system index terminal unit (the farthest valve from the pump) will experience a differential pressure lower than the minimum DeltaP specified at the current caliber position (i.e. 25kPa) it means the flow tolerance will be higher on that valve; instead all the others valves in the system will most probably experience a valve differential pressure higher than 35 kPa and therefore the energy saving benefit will not be affected significantly.

• Determine the pressure differential at the sensor location. Usually the sensor is placed at the distance from the pump equal to 2/3 of the distance of the farthest terminal from the pump itself. Set the pump speed to control such that the value indicated at the sensor is maintained constant under all conditions.

• Measure and record the total flow rate, pressure differential and energy consumption at the pump.

• Run all two port valves to their closed positions. Measure and record the total flow rate, pressure differential and energy consumption at the pump. Calculate and report the overall energy saving achieved i.e. between full load and minimum load operation.