How PICV Works? (In-Depth Explanation w/ Diagram)

PICVs are used in modern chilled water systems. They are fitted in AHUs and FCUs to regulate chilled water flow accurately. I have hands-on experience with PICVs. So, in this post, I’ll explain everything you need to know about PICVs.

PICV stands for pressure independent control valve. PICV is comprised of a valve and an actuator. The valve balances the flow and the actuator controls the flow. A pressure regulating mechanism is built within the valve to ensure the flow control is not affected by the pressure.

In some regions, PICV is referred to as PIBCV instead. The full form of PIBCV is pressure independent balancing and control valve. PIBCV better represents its function (as I’ll explain later) but globally, PICV is more common.

PICV Working Principle

PICV is a 2-in-1 type of valve that has the functions of 2 traditional valves: a balancing valve and a control valve.

Traditionally, chilled water systems use a balancing valve at each AHU and FCU to limit the maximum chilled water flow rate based on the design flow of the AHU and FCU. Then, a control valve that has an actuator (on/off or modulating) is installed at each AHU and FCU to regulate the chilled water flow based on the cooling demand (signal either from off coil temperature sensor or room thermostat).

The problem with this setup isthat the control valve is not pressure independent.

In a closed loop piping system like a chilled water circuit, when some of the traditional control valves are closed (due to reduced cooling demand), the pressure differential (ΔP) across other control valves increases. This could lead to several issues:

• Poor valve controllability & hunting: With higher ΔP, a small valve movement (open or close) will cause large flow swings. Room temperature overshoot/undershoot, oscillation and unstable control loops.
• Flow maldistribution: Nearer AHUs/FCUs “steal” flow during pressure spikes, starving other AHUs/FCUs that are farther away and creating hot/cold complaints.
• Balance doesn’t hold: The valve maximum chilled water flow rate setting (a.k.a water balancing) only works at the commissioning pressure, which is different from the actual operating pressure.
• Low ΔT syndrome at the plant: For the same space load, higher flow means lower water temperature. Plant ΔT and chiller plant efficiency drop (more pumps/chillers run to move extra water).

When a PICV is introduced, the balancing valve is eliminated. On top of that, the pressure regulating mechanism within the PICV ensures the ΔP is always constant, thereby keeping flow control accurate.

Preset Flow (Water Balancing)

Every PICV must be set to the AHU/FCU design flow so that the entire chilled water circuit is balanced. Otherwise, too much chilled water will flow through the coil, causing all sorts of problems as listed earlier.

Below diagram shows the internal mechanism of a PICV:

The green color component is the balancing mechanism. It can be manually set to a fixed maximum flow rate depending on the connecting AHU/FCU design flow using the “handwheel” at the bottom of the PICV as shown in the above diagram.

This flow setting is sometimes also known as the preset flow. Some PICVs preset flow is located at the top of the valve, where the actuator needs to be removed in order to access it.

For the above PICV, as we turn the handwheel to reduce the preset flow rate, the hole for water to flow through on the plate marked as 1b will gradually close. Once we’re done with the handwheel setting, the opening space for the water to flow through the valve will be fixed.

This is effectively the same working principle as a balancing valve. Water flow is physically restricted by the valve’s internal opening size and it has nothing to do with the actuator, yet.

Pressure Independent

Earlier in the diagram, I briefly explained how PICV achieves pressure independence. However, it can be difficult to understand. So, let’s run through some numbers.

Below schematic represents the pressure regulation of a PICV:

Assume that the flow control mechanism stabilises and:

• The inlet pressure (P1) is 10 bar.
• The pressure differential across the flow control mechanism (2) is 1 bar.
• The pressure differential across the balancing mechanism (1b) is 1 bar.
• The pressure differential across the pressure regulating mechanism (1a) is 1 bar.

As a result, the pressure at P2 is 8 bar and the pressure at P3 is 7 bar. The pressure differential between P1 and P2 is 2 bar, and the pressure differential between P1 and P3 is 3 bar.

Now, when the system dynamics change (eg: other PICVs closed, another pump is turned on, etc.), the pressure regulating mechanism automatically throttles or closes to increase its pressure differential. The resulting scenarios could be as follows:

• The inlet pressure (P1) increased to 12 bar.
• The pressure differential across the flow control mechanism (2) is still 1 bar.
• The pressure differential across the balancing mechanism (1b) is still 1 bar.
• The pressure differential across the pressure regulating mechanism (1a) increased to 3 bar.

Due to the throttling of the pressure regulating mechanism, the pressure at P2 is 10 bar and the pressure at P3 is 7 bar. As a result, the pressure differential between P1 and P2 is still 2 bar. But now, the pressure differential between P1 and P3 is 5 bar.

If the pressure regulating mechanism is not working, when P1 increases to 12 bar, the pressure differential across the flow control mechanism (2) and balancing mechanism (1b) could increase from 2 bar to 4 bar.

Based on basic fluid mechanics, increasing the pressure differential across a control valve will increase the flow, given the same control valve position. As such, the flow control is no longer accurate, which is experienced by a traditional balancing valve + control valve setup.

Basically, the pressure regulating mechanism automatically “absorbs” any pressure changes in order to maintain the pressure differential between P1 and P2 for accurate flow control. At the same time, the pressure differential between P1 and P3 needs to be within the control range of the PICV to maintain 100% valve authority.

Valve Authority

Valve authority (a) is traditionally known as the ratio between the differential pressure of a control valve at fully open position (ΔP_valve) and the sum of the differential pressure of the control valve at fully open position (ΔP_valve) plus the total differential pressure of other components the branch pipe (ΔP_branch), including pipe, strainer, coil, isolation valve, etc.

Valve Authority, a = ΔP_valve / ( ΔP_valve + ΔP_branch )

If the total differential pressure of other components in a branch pipe is 0.5 bar and the differential pressure of the control valve at fully open position (ΔP_valve) is 3 bar. The valve authority is:

Valve Authority, a = 3 / (0.5 + 3) = 85.7% (excellent)

But, if the total differential pressure of other components in a branch pipe is 5 bar instead, the valve authority drops to 37.5%. Logically, if other components in a branch pipe have a very high pressure loss (high ΔPbranch), how much water can flow through the branch is already capped. There’s very little that can be done by the control valve to increase flow.

In other words, a control valve must take a meaningful share of the total pressure drop in a branch pipe so that it has enough influence over how much water is flowing through a branch pipe and therefore, enable better flow control.

That’s for a traditional balancing valve + control valve setup, not for PICVs.

When we talk about valve authority, the ΔP_valve refers to the control valve because that is all it does. We can’t use ΔPvalve on a PICV because PICV is more than just a control valve. It also has a balancing mechanism and a pressure regulating mechanism, which induce pressure loss.

Regarding valve authority, what we want to know is how well a control valve can control the flow through a branch pipe. In other words, how much authority the control valve has over other components (strainer, coil, isolation valve, etc.) in terms of flow control (who’s in charge of the flow here?). We discuss it based on pressure differential because if the control valve has a much higher pressure differential than other components, it dictates how much flow is going through the branch pipe.

This is important for traditional control valves because it dictates their flow control accuracy.

Now, this kind of valve authority discussion does not apply to a PICV because a PICV will always have a constant differential pressure across its control valve (the flow control mechanism P1-P2).

Therefore, PICVs will always have predictable flow control (behaving like 100% valve authority), given that the operating differential pressure is within their control range.

Minimum Differential Pressure

Every PICV has a minimum and maximum differential pressure rating. The maximum differential pressure is to prevent cavitation and other issues due to high water velocity. The minimum differential pressure is to prevent losing its flow control accuracy.

Below diagram illustrates the minimum (start-up) differential pressure requirement of another PICV:

As mentioned earlier, a PICV must maintain a constant pressure differential across its flow control mechanism. In the case of the above PICV, the flow (Q) becomes constant only when the differential pressure P1-P2 is above a certain value. This threshold is the minimum differential pressure.

The differential pressure across the flow control mechanism of a PICV can be checked with the two measurement ports built on the valve using a manometer.

Other than brands and models, the minimum differential pressure of a PICV also depends on the preset flow (balancing) that I mentioned earlier. Below graph shows the minimum differential pressure at different preset flow (position):

Generally, the minimum differential pressure drops as we reduce the preset flow. While reducing the preset flow reduces the differential pressure requirement, it introduces higher flow deviation.

Below graph shows the flow deviation % against the preset flow:

Assume the preset position is available from 0-10, which corresponds to 0 to 1000 L/hr of flow rate. Adjusting the preset position to 5 means the desired flow rate is 500 L/hr. This is 50% adjusted flow.

At 50% adjusted flow, the flow deviation is ±15%. Meaning, the actual flow varies between 425 and 575 L/hr. In other words, the more you adjust the flow (lowering the preset position), the lower the flow accuracy.

That’s why oversizing is not desirable. If you need 500 L/hr of flow, choose a PICV that has a maximum flow of something around 600 L/hr, giving yourself some buffer while not needing to adjust the preset flow more than necessary.

PICV Actuator

A PICV has two components. One is the valve, which is what we’ve discussed mostly so far. The other component is the actuator, the motor or device that drives the flow control mechanism.

In the earlier PICV cut section diagram, the center rod of the blue color flow control mechanism is called the valve stem. The actuator sits on top of the PICV valve and its stroke is in contact with the valve stem. This is a typical construction of a PICV with an electromotive actuator.

Below diagram shows the placement of the actuator stroke on the valve stem of another PICV:

The light blue color is the valve stem, connecting the dark blue color flow control mechanism. The light purple color is the actuator stroke, driven by the dark purple color actuator motor.

Without the actuator, the valve stem naturally stays at the highest position (fully open) due to the internal spring. When equipped with an actuator, the actuator motor drives the stroke down, pressing the valve stem down, thereby closing the valve.

Some actuators allow you to see their stroke movement through an indicator. Below short video shows the actuator stroke indicator move down as the valve closes:

Depending on the valve stem length, the actuator stroke indicator may not go all the way down. Each actuator must be calibrated to its valve stem length, which I’ll explain later in PICV Setting.

On/Off vs Modulating

There are two types of actuator controls: a) 0-10V modulating and b) on/off.

Modulating actuators will drive the valve stem down in stages. For instance, 10V is fully open, 8V is 80% open, 3V is 30% open and so on.

How much signal voltage is sent to the actuator depends on the FCU and thermostat. It could be something like 1V for every 0.5°C temperature difference between the room and the setpoint. For instance, room temperature is 26°C, setpoint temperature is 23°C, delta T is 3°C and thus, 6V signal to the actuator and 60% valve opening position.

Such a control method is also known as linear characteristic or proportional control. Basically, every 0.5°C temperature difference always corresponds to 1V signal.

On the other hand, on/off actuator is simpler. Whenever the room temperature deviates from the setpoint temperature, the actuator retracts to open the valve and vice versa. The activation trigger could be 1°C deviation, meaning if the room temperature is 1°C higher than the setpoint temperature, the valve opens. If the room temperature is 1°C below the setpoint temperature, the valve closes.

In comparison, FCUs with a modulating actuator are like inverter air conditioners, able to minimize the room temperature fluctuation for better comfort. Meanwhile, FCUs with an on/off actuator behave like traditional non-inverter air conditioners with slight temperature fluctuation, which may not be noticeable in smaller-capacity systems.

Fail-Safe

In the event of an emergency where power to the actuator is cut due to various reasons, some actuators have a feature called fail-safe. Upon no power, the actuator can either fully open or fully close the valve, depending on the application requirement. This includes when the power module of the actuator itself is faulty.

For example, in a production plant, maintaining cooling is crucial. Hence, the fail-safe feature can be set to fully open whenever there is an emergency power cut or the actuator itself fails.

Actuators with a fail-safe feature typically use capacitors (like battery) to store the electricity. This is how they can still move the stroke and close the valve even when there’s no power.

PICV Setting

PICVs are designed to work within a certain range of flow. Hence, they need to be set according to the application requirements (eg: AHU/FCU design flow, fail-safe, 0-10V or 2-10V, etc.).

As mentioned earlier, the valve maximum flow rate can be set using the handwheel to the FCU design flow rate. On top of that, the actuator must also be set to the same flow rate as the valve. Out of the two, whichever one with the lower flow setting will be the flow restrictor.

Electromotive actuators are designed to work with different valves. Hence, their stroke needs to be calibrated based on the stem length of that particular valve.

This is often done by powering on the actuator for the very first time after mounted on the corresponding valve. Actuators typically run a calibration procedure when powered on for the very first time at the site.

If this calibration is not done, the actuator stroke could extend beyond the valve stem limit and cause internal damage. Or, the actuator stroke could retract beyond the valve stem’s highest position, resulting in poor water flow control.

The calibration of PICV actuators is critical but often overlooked at the construction site.

Some installers may separately test the actuator’s functionality by powering on them without mounting them on the valve and later, never recalibrate them, as some actuators require factory reset to re-engage the calibration procedure.

PICV Selection

PICVs are selected based on the AHU/FCU flow rate requirement. As I explained earlier, oversizing is not desirable as it affects the flow control accuracy.

Naturally, PICVs are sensitive to dirt, which means the water quality must be good prior to commissioning. Flushing bypass pipes and side stream filters are part of the consideration when choosing to use PICVs.

Some PICVs have a better design that enables them to resist dirt up to a certain extent. Nonetheless, fine particles circulating in the chilled water loop will eventually cause some of the PICVs to become stuck or perform inconsistently as the internal build of the valve is delicate.

Hence, we need to decide whether or not we should use modulating or on/off. Is fail-safe necessary given our application? And, do we have enough budget for a good side-stream filter and the subsequent chemical dosing, which is an ongoing process after commissioning?

As the world is moving towards a more green and sustainable future, PICVs are one of the key contributors to an overall better energy efficiency system.