The Valve That Leaks Through: How Passing and Stuck Valves Quietly Waste Energy and Undermine Redundancy

• A valve is passing, also called bypassing or leaking through, when fluid moves through it while it is closed.
• Worn seats, debris, corrosion, heat distortion, a drifted actuator, a slipped linkage or an actuator too weak to close against pump pressure all cause it.
• In chilled water, passing valves cause low return temperatures and low delta T; in heating, they cause simultaneous heating and cooling.
• Ultrasound readings upstream, on the body and downstream of a closed valve are the standard field test. Downstream louder than upstream means likely passing.

A passing valve is a valve that lets fluid through while it is closed. It is also called bypassing or leaking through.

Unlike the external leaks covered in Part 1 and Part 2, a passing valve leaves no water on the floor. The leak is entirely inside the pipe.

That makes it one of the hardest faults to notice. The valve reads closed, the controls believe it is closed, and the system behaves as if something else is wrong.

Passing valves turn up in chilled water, condenser water, heating water and steam systems. Their effects differ, but the cause is the same: a seal that no longer seals.

A closed valve lets flow through when its seat and disc no longer meet tightly. Something has worn, blocked or distorted the sealing surfaces.

• Worn or eroded seats and discs from years of throttling and flow.
• Debris or scale holding the valve off its seat.
• Corrosion of the sealing surfaces.
• Heat distortion, where temperature warps the body or seat.
• Incomplete seating, where the valve is not driven fully closed.

Once a small leak path exists, flow through it erodes it further. A tiny bypass tends to grow, especially across valves that see a large pressure difference.

A control valve can read closed and still be open because the controller's command and the valve's actual position have drifted apart. The controller says 0%; the valve is not fully shut.

• Drifted actuator stroke, so 0% on the controller is not fully closed.
• A slipped linkage on the shaft, so the actuator moves but the valve does not follow fully.
• A close off rating too low for the system: the actuator cannot close the valve against the pump's differential pressure.

The close off rating is the pressure difference a valve and actuator can close against. If the pump pushes harder than that, the valve is held slightly open no matter what the controller commands.

Comparing the actuator's position indicator with the valve's own position mark is a quick check. When they disagree, the linkage has slipped or the stroke has drifted.

In a chilled water plant, a passing valve sends water where it should not go, which lowers return water temperature or raises supply temperature. Both waste energy.

Passing bypass and coil valves

A passing bypass or coil valve sends supply water straight to return. That lowers return water temperature, a cause of low delta T syndrome, where the temperature difference between supply and return is lower than design.

Low delta T means the plant pumps more water to deliver the same cooling, and chillers run less efficiently. It is covered in the Cooling Plant Performance sub-series.

Passing valves are among the hardest causes of low delta T to find, because they hide in plain sight. Every coil valve in a building can read closed while a handful of them quietly pass supply water to return. A plant whose delta T has drifted down over the years often has several.

Passing isolation valves on idle chillers

A passing isolation valve on an idle chiller lets warm water bypass through the idle machine and mix into the supply. That raises supply temperature, and the running chillers work harder to make up for it, as covered in Rotating Equipment Part 7.

In a heating system, a passing valve adds heat that nobody asked for. The worst case is simultaneous heating and cooling.

A passing heating valve on an air handler adds heat to air that is also being cooled. Both systems run, fighting each other, and both waste energy.

The symptom is often a zone that will not cool in summer. Or an air handler's cooling valve sits wide open while the space stays warm. The heating valve reads closed, so it is the last thing anyone checks.

Steam valves that pass when closed overheat spaces and waste steam in the same way, covered in Part 8.

A leaking check valve on an idle standby pump lets flow from the running pump circulate backward through the idle one. The standby pump can visibly spin backward.

That backward flow robs flow from the system. The running pump works harder, and less water reaches the load.

It also puts the standby pump at risk. Starting a pump while it turns the wrong way loads the motor and coupling hard. The pump may also fail to take load when it is needed most.

An idle pump with a turning shaft or coupling is directly observable on a walk past. It is one of the few passing valve faults anyone can see without instruments, and it is worth looking for on every round. Standby pump health more broadly is covered in Rotating Equipment Part 1.

Ultrasound detects a passing valve by hearing the turbulence of flow squeezing through a narrow gap. Flow through a small opening creates high frequency turbulence, even when the flow is small.

The standard field method takes readings upstream, on the valve body, and downstream, then compares them (US Patent 6,804,992). Airborne and contact ultrasound both apply.

• Downstream higher than upstream: the valve is likely passing.
• Downstream equal to or lower than upstream: the valve is likely sealed.

Measurements must be taken at consistent points to compare over time (Pumps and Systems). Marking the test points on the pipe makes repeat readings meaningful, because a reading taken a few inches away can differ.

The upstream reading is the reference. It accounts for noise traveling along the pipe from pumps and other valves, so the comparison isolates what the valve itself is doing.

A thermal camera can sometimes find a passing valve, but it is less reliable than ultrasound. A temperature difference downstream of a closed valve can indicate leak through.

Several effects can mislead it (IPIECA):

• Residual heat from recent valve cycling.
• Natural convection inside large pipes, moving warm water without real flow.
• Conduction along the pipe, warming the downstream section by contact.

A leak in a saturated system may not raise downstream temperature at all, while it still makes an ultrasonic signature (POWER magazine). That is why thermal works best as a first screen, with ultrasound to confirm.

Valves that will not fully open cause the opposite problem: starved flow. The symptom is a higher than expected pressure drop across the valve and a coil or branch that cannot meet load.

• A failed actuator that no longer drives the valve fully open.
• A slipped linkage, so the actuator moves but the valve does not.
• Debris lodged in the valve.
• A manual balancing or isolation valve left partly closed after service.

The last one is common and easy to fix. A valve closed for a repair and only partly reopened can starve a coil for years. Pressure gauges across the valve and the valve's position mark show it.

You check a control valve's stroke by commanding it fully closed and fully open, and confirming the valve itself follows. The aim is to prove that 0% means shut and 100% means open.

1. 1. Command the valve to 0% from the controller or building automation system.
2. 2. Confirm the actuator display, the actuator indicator and the valve's own position mark all show closed.
3. 3. Listen with ultrasound downstream and compare with upstream to confirm it has sealed.
4. 4. Command 100% and confirm the valve reaches full open on its mark.
5. 5. Note any lag, stall or difference between command and position, and return the valve to normal control.

A valve that reaches its closed mark but still passes has a seat problem. A valve that never reaches its mark has an actuator, linkage or close off problem.

Stroke checks are quick and need no shutdown. Doing them once a year on the valves that matter catches most drift before it costs much.

Isolation valves should be exercised because a valve that never moves is a valve nobody knows will seal. Scale and corrosion build on seats and stems over years in one position.

Exercising a valve, closing and reopening it on a schedule, keeps the stem free and shows whether the valve still seals. The time to discover a seized or passing isolation valve is during a planned test, not during an emergency.

Exercising has one known side effect: stem packing can weep after the valve moves, as covered in Part 2. Checking the stem after each exercise turns that into a routine look, not a surprise.

You survey a plant for passing valves by listing the valves that should be closed, then testing each one with ultrasound and checking its position. Start with the valves that matter most for energy and redundancy.

1. 1. List every valve that should be closed right now: idle chiller and pump isolation, bypasses, crossovers and closed control valves.
2. 2. Mark consistent test points upstream, on the body and downstream of each.
3. 3. Take ultrasound readings at those points and compare downstream with upstream.
4. 4. Check pipe temperatures either side, as a supporting screen.
5. 5. Compare each control valve's commanded position with its actuator display and valve mark.
6. 6. Look at every idle pump for a turning shaft or coupling.
7. 7. Record results and repeat on a schedule, using the same points.

Valves on standby chillers and pumps deserve priority. A passing valve there costs energy every day and redundancy on the one day it is needed.

A passing valve costs energy in every building. In a data center it can also cost redundancy.

In an office building

Low delta T, extra chiller and pump energy, simultaneous heating and cooling, and comfort complaints are the main costs. They are often blamed on controls or equipment when the valve is the cause. Valve checks belong in routine heating, ventilation and air conditioning (HVAC) maintenance.

In a data center

A passing isolation valve on a standby chiller or pump undermines redundancy. A leaking check valve can let a standby pump spin backward and fail to take load when needed.

Redundancy that has never been tested under real conditions is an assumption. A passing valve is one of the most common reasons that assumption turns out to be wrong.

Several normal behaviors look like a passing valve. Knowing them prevents replacing good valves.

• A normal minimum flow bypass that is meant to pass water.
• Heat conducting along a pipe from a hot header, which warms a downstream section with no flow.
• A three way valve working as designed, which always passes flow through one port or the other.

Checking the design intent first saves time. A valve that passes flow because it is supposed to is not a fault. The design drawings and sequence of operations show which valves are meant to bypass and which are meant to seal.

This is why no single sensor is enough. Ultrasound, temperature, position indicators and the design drawings each rule out a different explanation.

A walk past can prove a pump spinning backward, an actuator that disagrees with its valve, and a manual valve left partly closed. It cannot measure how much a valve is passing.

• Directly observable: an idle pump spinning backward, an actuator position that disagrees with the valve's position indicator, and a manual valve left partly closed.
• Strong indicator: downstream ultrasound higher than upstream on a closed valve, and warm water downstream of a closed chilled water valve in steady operation.
• Suspected: the leak through rate. That needs a flow measurement or isolation test.

That is the limit of any route based check. It finds the passing valve; a flow or isolation test measures it.

Routine visual inspection of position marks and idle pumps, and a walkthrough record of ultrasound readings at fixed points, turn these checks into a trend. That is the core of predictive maintenance.

• Idle pumps for shafts or couplings turning.
• Actuator indicators compared with valve position marks.
• Manual valves that are partly closed and should not be.
• Pipe temperatures upstream and downstream of closed isolation valves on idle chillers and pumps.
• Controller commanded position compared with the actuator display.

Coming Soon

1. 1. Which valves on your site are supposed to be closed right now?
2. 2. When was each control valve actuator last stroked and checked against its position indicator?
3. 3. Do any idle pumps turn on their own?
4. 4. Is your chilled water delta T lower than design?
5. 5. Are the close off ratings of your control valves matched to your pump heads?

What is a passing valve?

A passing valve lets fluid through while it is closed, leaking internally rather than into the room. It is also called bypassing or leaking through.

How do you know if a closed valve is leaking through?

The standard field test compares ultrasound readings upstream, on the body and downstream of the closed valve. If downstream is higher than upstream, the valve is likely passing. Warm water downstream of a closed chilled water valve in steady operation is another sign.

Can ultrasound detect internal valve leaks?

Yes. Flow through a narrow gap creates high frequency turbulence that ultrasound picks up. Readings must be taken at consistent points so they can be compared over time.

Can a thermal camera find a passing valve?

Sometimes, but it can mislead. Residual heat, natural convection and conduction along the pipe can all cause false readings, and a leak in a saturated system may not change temperature at all. Use thermal as a screen and ultrasound to confirm.

Why does a control valve leak when it reads closed?

The actuator stroke may have drifted, the linkage may have slipped, or the actuator may not be strong enough to close against the pump's differential pressure. In each case the controller says closed while the valve is slightly open.

What is a valve close off rating?

The close off rating is the pressure difference a valve and actuator can close against. If the pump's differential pressure exceeds it, the valve cannot fully close and will pass flow.

Why is my standby pump spinning backward?

Its check valve is leaking, so flow from the running pump circulates backward through the idle one. This robs flow from the system and can damage the pump if it is started while turning backward.

How does a passing valve cause low delta T?

A passing bypass or coil valve sends supply water straight to return, lowering return water temperature. The difference between supply and return falls below design, so the plant pumps more water for the same cooling.

What causes simultaneous heating and cooling?

A common cause is a passing heating valve on an air handler, adding heat to air that is also being cooled. Both systems run against each other, wasting energy.

• Ultrasonic Sensing in Data Centers: Hearing Faults Before They Fail
• Reading the Gauges Nobody Monitors: What Your Facility Already Knows and Never Writes Down
• Bearing Failure, Stage by Stage: Why Ultrasound Hears It Before Vibration Feels It and Heat Shows It
• Steam Traps and Steam Leaks: How Failed Traps Waste Energy, Cause Water Hammer and Hide in Plain Sight

• US Patent 6,804,992: upstream versus downstream ultrasound comparison
• Pumps and Systems: ultrasound and thermography on valves
• POWER magazine: valve leak cost and thermal limits
• IPIECA: passing valves and thermal method pitfalls