FREQUENTLY ASKED QUESTIONS about PICV valves. Also there is another article about how PICV works and it’s advantages in this link.
GENERAL VALVE QUESTIONS
How PICVs work?
Hot/chilled water or 50% water-glycol solutions in closed loop systems enter the valve and pass through a variable rectangular adjustable flow limiter (C) that is connected to the ring with the dial (A) for presetting the maximum volumetric flow.
The medium then flows through the temperature control valve (D) with a linear characteristic and a stroke of 5 or 6.5 mm. A powerful spring (B) ensures safe closing.
Before leaving the valve, the medium passes through a second throttling point (E) controlled by the built-in mechanical automatic differential pressure regulator (F).
This regulator is the heart of the valve and ensures that the selected volumetric flow is maintained across the whole differential pressure working range
- A. Presetting dial
- B. Closing spring
- C. Throttling point linked to A
- D. Control seat and plug
- E. Pressure regulator’s throttling point
- F. Differential pressure regulator
What is a PICV?
A Pressure Independent Control Valve is the combination of a differential pressure regulator (1), a 2-way control valve (2) and a settable flow rate adjustment (presetting) (3). It results in a self-balancing control valve. The differential pressure regulator maintains constant the differential pressure across the control valve, resulting in the highest authority (close to 100%). The presetting adjusts the maximum flow rate across the valve. The control valve modulates the flow rate according to system requirements if an actuator is mounted; in case the control valve is characterized, it can be called PICCV; in case the control valve has a standard linear or quick acting characteristic, it is generally called PICV. It is suitable for treated water. In order to provide maximum energy efficiency and whole system pressure independency, a PICV must be fit on every terminal unit.
What is a PICV used for?
A PICV is a control and balancing valve. When installed on a terminal unit, it ensures every time the right flow rate, when required, irrespective of the pressure fluctuation in the water distributor networks. Balancing is always guaranteed, maximizing the energy efficiency. It saves 70% pump energy consumption compared to 4-way control valves.
Why should I use a PICV?
PICV are the state-of-the-art of the technology of hydronic balancing and control valves. They make the water distribution network pressure independent (if installed on every terminal unit). They are self-balancing valves and allows for fully variable flow systems, always dynamically balanced irrespective of the thermal load. They reduce designing time and costs: control valve authority calculation and branch pressure drop calculation are not needed anymore. Designer can focus on total system pressure (pump head) calculation. As they are self-balancing valves, commissioning operation are reduced to one main action: technicians just adjust the presetting dial n suitable value. Then, differential pressure across each valve should be checked. The client gets exceptional energy savings and perfect thermal comfort: the PICV provides the required flow rate to each terminal unit every time, minimizing the flow rate and pumping costs. Especially with Equal Percentage PICV, overflow, overcooling/heating and Low DeltaT syndrome disappear. PICV save 70% pump energy consumption with respect to constant flow systems and 30% with respect to DPCV.
What is the start-up pressure and why is it important?
The start-up pressure is the minimum differential pressure across the PICV (P1-P3.1) which is required to keep the flow rate constant. Over the start-up pressure, the PICV maintains the flow rate constant; under the start-up pressure, the PICV works as a standard 2-way valve. It is important because it is the parameter to check whether the valve is properly working.
What is the hysteresis and how can it affect the system performance?
The hysteresis is the maximum difference of flow rate between the downward (differential pressure decreasing) and the upward (differential pressure increasing) curves within the minimum and maximum working differential pressure. It affects the readings of flow rate during the commissioning and, moreover, it influences the real flow rate flowing through terminal units. Smaller the hysteresys, more stable and precise the flow rate through units.
What is the Differential Pressure Regulator and which types are there?
The differential pressure regulator (1) is the dynamic component of a PICV. It is made by a rubber membrane, a spring and sleeve. The differential pressure on membrane faces and the spring provide with a balance, resulting in constant differential pressure across the control valve. Pressure fluctuations are absorbed by the membrane: it drives the sleeve opening or closing a window, which limits the flow rate. There is also cartridge type differential pressure regulator (also called insert) which are similar to automatic balancing valve cartridge.
What is the PICV presetting and how can PICV be set to correct flow?
The presetting dial of a PICV is a device, with a numbered scale, which adjusts the maximum flow rate of a PICV. It can limit the control valve stroke or a dedicated passage. The presetting is internal in case the actuator or the manual adjusting cap covers the dial or external if it is accessible even with the actuator or the cap onto the valve. The PICV is set by turning the presetting dial to the suitable position, according to table or calculation settings.
Are PICV suitable for dirty water and is there any PICV which endure with dirty water?
Water quality affects PICV performances. Debris and impurities which come from installation and corrosion, are carried around the system by the water and they gather around moving parts and O-Rings. Accumulation of such material blocks the free motion of the differential pressure regulator. In case the differential pressure regulator gets stuck, the PICV works as a standard 2-way control valve and it cannot maintain constant the flow rate anymore. All national standards regulate the water quality, limiting the content of impurities, specifically iron oxide. For these reasons, several actions are strongly suggested: installation of flushing by-pass to flush out the system before operation; installation of magnetic dirt separators; installation and maintenance of water treatment plants.
How can I maintain a PICV?
A PICV can be maintained if its internal components can be replaced or cleaned without removing the valve from pipes or damaging the valve. Components that should be maintainable in PICV are the differential pressure regulator and the control valve.
How can I size a PICV?
All PICV must be selected based on terminal unit design flow rate firstly. The right PICV should have the closest higher flow rate with respect to the terminal unit design flow rate. The PICV should work with the highest presetting position possible. In order to reduce the minimum differential pressure, a small safety margin (5-10%) can be taken into consideration while selecting the PICV.
Can I install electric actuators on PICV?
Definitely yes. A PICV is a water flow rate control valve and every valve has a connection for installing an actuator. Axial valves must work with axial actuator (thermoelectrical and electromechanical) while rotary valves must work with rotary actuators. Actuator stroke should always match control valve stroke to enhance modulation accuracy.
What is the flow rate control characteristic of a PICV?
The flow rate control characteristic of a PICV is the correlation between the valve opening position and the flow rate flowing through the valve. Both parameters can be normalized referring to maximum valve opening position and maximum flow rate (control valve fully open) at given presetting. The valve opening position is directly proportional to a modulating control signal sent to an electric actuator. Available characteristics are: QUICK OPENING, LINEAR, EQUAL PERCENTAGE.
QICK ACTING valve characteristic curve (PICV)
LINEAR valve characteristic curve (PICV)
EQUAL PERCENTAGE control valve characteristic curve (PICCV)
Which kind of control characteristic is advisable for FCU and AHU?
Considering that all coils (water-to-air heat exchangers) have a parabolic characteristic (correlation between water flow rate across the coil and thermal power output), the better flow rate control characteristic is the one mirroring the coil characteristic. This characteristic is the EQUAL PERCENTAGE one. Thanks to this, the whole air temperature control system which includes sensors, the controller, the actuator, the PICCV and the coil is linear. A linear system is the most general and easiest to control.
Controller asks for 50% thermal power, it sends a 5V control signal to actuator, actuator closes 50% the control valve.
What do full stroke and reduced stroke mean?
Full stroke control valve means that the PICV control valve does not change its stroke (travel length) whatever the presetting is. The maximum flow rate limitation (presetting) is carried out by limiting the passage of a dedicated window. Reduced stroke means that the PICV control valve limits the stroke in order for the presetting. Full stroke control valve has better control ability whatever the presetting is and copes every time with suitable actuators.
What are the available certifications for PICV?
Unfortunately, there is no international certification body which developed a certification standard for PICV. The CE marking on PICV is regulated by the Pressure Equipment Directive of the European Union. At the time, the only activity conducted by a certified institute is the performance testing by BSRIA: it can verify and prove the PICV accuracy, the leakage rate, the hysteresis and, above all, the control characteristic. Therefore, BISRIA can verify for instance if the equipercentage characteristic is always provided by the control valve whatever the flow setting is.
SYSTEM AND DESIGN QUESTIONS
Do I have to install other balancing and control valve along with PICV?
No, you do not. The PICV dynamically balances the loop where it is installed: if all the terminal units (FCU, AHU, bypasses, Heat Exchangers, radiators, future loops) have a PICV, all terminals are always balanced; therefore, risers, main pipes and branches are consequently balanced. Additional DRV and DPCV should not be installed. Similarly, additional control valves should not be installed, except for water recirculation on end of lines (if required).
Should I install Double Regulating Valves DRV at risers?
No, you do not. Balancing is provided by the PICV. DRV generates higher pressure drop only and it can affect the startup pressure.
Can PICV be installed in series?
No, you cannot. Each PICV is designed to maintain the flow rate constant. Each PICV needs at least the start-up differential pressure to work properly. On specific conditions, both PICV can work and affect each other. Sometimes the upstream PICV can potentially limit the total flow rate so that the downstream one cannot regulate the flow rate.
Can I install a 4-port control valve as end-of-line by-pass with no PICV?
No, you should not. In case a 4-port control valve is install on end-of-line for water recirculation, a PICV must be install upstream the control valve as dynamic flow limiter. This way, both terminal unit and by-pass are always dynamically balanced. The PICV should never be install between the 4-port control valve and the terminal unit: indeed, the by-pass loop would be unbalanced and this would affect the start-up pressure of PICV nearby.
Can a PICV be used with 2-port control valve?
The flow rate control must be done by the PICV. In case the flow rate control is carried out by another 2-way control valve and the PICV is placed as flow limiter, the authority of the 2-way control valve is very low: the PICV keeps constant the flow rate and limits flow rate variations. The control is almost ON-OFF.
Why control characteristic is important for energy saving?
The flow characteristic influences the system performances and the comfort. The control valve flow characteristic must match the terminal unit power characteristic in order to minimize the flow rate at every load level. The load level is determined by the system controller through different sensors. The controller logic is commonly linear: so the control valve characteristic should always mirror the terminal unit power characteristic, minimizing the flow rate and consequently maximizing the energy consumption.
Should strainer be fitted upstream a PICV?
Yes. It is highly recommended in order to avoid any big particle through the PICV. A standard mesh FM028 is enough. Some companies recommend the installation of a Filterball valve upstream the PICV: it is a ball valve with integrated strainer which protects the PICV and beside it works as an isolation valve.
Should I install a flow measurement device to each terminal unit?
Yes, it is highly recommended. A flow measurement device like a Venturi allows very accurate (±3%) and repeatable flow readings. Any flow measurement through PICV test point is not accurate.
Can PICV be backflushed/reverse flushed?
No, it cannot. Any flush through the PICV must be avoided: the flushing would bring dirt and debris inside the PICV, getting movable components stuck. Moreover, reverse flushing can also fill channels and holes with debris, making the valve unusable.
Where is the best location for the differential pressure transducer?
The best location of the differential pressure transducer to achieve the highest energy performance is the index circuit. It should be set in order to keep the start-up pressure across the PICV located at the index. In case of multi riser system and unpredictable pressure layout, a transducer should be put on the least favored loop of each riser.
How can I calculate the pressure drop across a PICV?
As the PICV is a dynamic balancing valve, the differential pressure across the valve cannot be prior calculated: it can change time by time according to instantaneous system configuration. For pump head calculation, the differential pressure to be considered at the index circuit is the start-up pressure of the selected PICV plus a small safety margin.
The definition of control valve authority is the ratio between the differential pressure across the control valve and the differential pressure of the whole section controlled by the valve. As per PICV design, the control valve of a PICV controls the flow rate over a differential pressure which is kept constant by the differential pressure regulator. So, the ratio is always almost 1.
Can I install PICV on drinking water systems and open circuits?
No, you cannot. Pressure Independent Control Valves are specifically designed to control and balance closed circuits for HVAC purposes, where water is properly treated. Uncontrolled water hardness can affect moving parts of the valve. Moreover, some materials in the valves could release substances into the water: no migration test has been carried out.
What are the two test points for on a PICV?
Typically, the two test points on a PICV are for measuring the differential pressure across the PICV. The reading can be compared to the start-up pressure at the selected presetting (given by the PICV manufacturer). Through this, the technician can know if the valve is working as a pressure independent valve.
How do I know if a PICV is working?
Through the pressure ports. By using a differential pressure manometer, the differential pressure across the entire PICV can be checked: in case the DeltaP is higher than the start-up pressure at the given presetting, the PICV is working and keeping constant the flow rate.
Can I optimize pump operating speed with a PICV?
Yes, you can with Pettinaroli PICV. The most efficient max pump speed is the one ensuring the start-up pressure at the index circuit and full load condition (all terminal units open). While measuring the DeltaP across the index PICV, the max pump speed can be set by reducing step by step the speed until the start-up pressure (plus a safety margin) is reached at index. Then, the differential pressure transducer, placed at the index, shall be set accordingly.
Can I measure the flow rate across a PICV?
Usually not. Few PICV in the market allow flow rate measurement through the pressure ports but the stated accuracy is very low. It is not possible on some PICV models. some companies suggest the installation of a Venturi flow measurement device with a calibrated orifice: the overall pressure drop is extremely limited and the measurement accuracy extremely high (±3%).
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