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- EXTRA EDITION
Refrigeration control valves for industrial refrigeration systems have been an important system component since the 1920s.
When a control valve fails, the result can be as significant as a more costly system component failure, such as a compressor or condenser. Yet when system maintenance is the topic of discussion, few operators consider the condition of the control valves.
This article describes areas to review when a control valve has failed, procedures to follow to safely install and repair control valves, and system conditions that may affect valve performance.
Most industrial refrigeration control valves and solenoid valves operate on the same basic principles. Thus, the information presented can generally apply to most manufacturers’ control valves in our industry. However, it is advisable — and for safety reasons, necessary — to read the information provided with any manufacturer’s equipment. If questions arise, contact the manufacturer for additional information.
Getting The System Ready for Valve ServiceNormally, most control valves must be “pumped out” or evacuated of any refrigerant before the valve can be serviced.
When servicing a control valve, be sure the remainder of the system upstream or downstream of the valve is not adversely affected when the control valve is evacuated. This can require anything from a total system shutdown to simply opening a bypass line.
As an example, repairing a valve in the discharge line to a condenser may require shutdown of part or all of a plant’s refrigeration. Whenever isolating a control valve for service, always manually open the valve to make sure refrigerant at the valve inlet and outlet is evacuated before servicing.
Solenoid-operated valves should be manually opened and energized for a short time to be sure the pilot section of the valve is evacuated.
Always tag any isolation valves against accidental closure or opening. Always tag and lock out any electrical control circuits to prevent power from accidentally being turned back on. Always be sure liquid lines are evacuated before isolating sections so liquid expansion will not occur. This caution also applies to refrigerant vapor lines or oil lines.
Beware of strainers and sections of piping that may trap liquid refrigerant or residual oil and may require a longer time to evacuate. Caution must be taken whenever removing a seal cap from either a control valve or hand valve as refrigerant may be trapped in the seal cap and escape during its removal.
Always follow accepted safety practices when servicing any part of a refrigeration system. These include — but are not limited to — using safety glasses or a face shield, and having protective equipment and personnel trained in its use readily available. Also, identify locations for cold-water eye-wash stations, exits, and fire extinguishers.
Installation NotesDuring installation or servicing, keep valves clean and free from dirt and moisture. When provided, be sure to leave plastic protectors in place until installation.
Always allow proper clearance for installing valves. Do not use the valve to “stretch” or “align” pipe. Using flange bolts to close a large gap can distort or stress the valve. This can cause the valve to malfunction or the flange bolts to fail.
Always be sure the valves and piping are properly supported to avoid stressing the valve or piping. Be sure to follow the valve manufacturer’s bolt torque specifications for tightening bolts and nuts.
Avoid installing valves where they may be damaged by material handling or other equipment. Be sure to provide for clearances necessary to service control valves and strainers. This information is normally provided in the manufacturer’s service bulletin.
Check to be sure the valve is installed in the proper direction of flow. Most solenoids and pressure regulators will allow reverse flow if a higher pressure downstream (than upstream) develops. Check valves at the outlet of these valves are required to prevent reverse flow through a regulator or solenoid.
Caution: Never install a check valve at the inlet of a solenoid or pressure regulator with electric shut-off. The consequences of trapping liquid between the two valves may be dangerous and destructive.
Once we are certain that the valve is evacuated of refrigerant and reduced in pressure, the valve is ready to be opened to atmosphere.
Valve TeardownA simple set of procedures should be followed to tear down a control valve.
When a control valve is insulated, remove the insulation carefully by cutting it in pieces that can be reused and repaired. If the control valve is a solenoid or is solenoid operated, be sure the coil is de-energized and cannot energize while off of the solenoid bonnet tube.
If the valve is a pressure regulator, back off the adjusting stem so there is no longer tension on the bonnet spring. This is not required on valves that have sealed pressure pilot assemblies, since the pilot assemblies cannot be disassembled or repaired and must be replaced.
If the valve is to be repaired and returned to service, be sure to have a new set of flange bolts on hand and a gasket kit. Flange bolts may have to be cut off to remove the valve from the line.
Having a gasket kit on hand will allow you to determine the problem with the valve and re-install it as may be necessary until the proper repair parts are available. Many systems have a backup or “swing” compressor in case a compressor fails. It would make sense to determine those valves that are extremely critical to system operation in order to have replacements on hand should a failure occur.
Many control valves available today can be converted into a number of variations. Having a few components on hand may allow one valve to back up several similar devices in different parts of a system.
Take the time to investigate the versatility of the valves installed. This could avoid a costly problem, not to mention saving time should a failure occur.
Always have the valve service bulletin on hand as a reference if questions should arise during teardown or assembly. Control valve service bulletins usually have a troubleshooting guide to help you determine the cause of a problem or failure.
Common ProblemsSome of the more common problems encountered include:
Symptoms: Pressure fluctuates or “swings,” the control valve chatters. Cause(s): The valve is oversized, the actual load has decreased to a condition less than the valve’s minimum capacity, or the control valve was selected at a pressure drop lower than actual operating conditions.
Symptoms: Pressure difference across the control valve does not change even when it is manually wide open. Pressure setting on regulators can only be made above a certain setpoint. Cause(s): The valve is undersized, the actual load is much greater than what the control valve was selected for.
Cause(s): The pilot seat or main valve seat has dirt on it, is eroded, or has an obstruction on it that prevents the seat from closing. The pilot passages are obstructed. The diaphragm is cracked and the valve is in the wrong control pressure range for the application. The piston is jammed due to dirt. The piston and/or piston bore may be worn to a point where the diametrical clearance is too large.
The latter condition could also cause an excessive pressure drop across the valve during operation. In some instances, the pressure drop across the control valve may not be high enough to open or keep the valve open. Solenoid valves may also have a coil shorted or a fuse may be blown.
Damaged or worn parts can also assist in determining system problems and causes of valve failure. Worn pistons, valve port plugs, and stems that appear to have a dull, sand-blasted surface are generally indicative of parts exposed to flashing liquid. This condition can also cause pistons to wear below accepted diametrical tolerances. When this occurs, excessive pilot gas can blow past the piston, requiring a higher-pressure drop to open.
This wear will occur when the proper conditions exist to cause liquid to flash or expand. This condition can cause premature wear or failure of valve internal parts. Generally, subcooling the liquid and/or reducing the pressure difference through the valve can help to reduce this problem.
Cracked diaphragms are usually indicators of over-pressurizing a valve. Or, this can be a sign of an undersized valve causing the diaphragm to work very close to the pilot seat. This can set up a high-frequency vibration or resonance that will stress the diaphragm material and cause it to fail.
The solution to this problem is to create conditions that allow the valve seat to operate in a more open position. This can be accomplished by either lowering the pressure drop across the valve or reducing the valve capacity by installing reduced capacity main valve assemblies.
When piston stems or opening stems appear to be “mushroomed,” this is usually an indication of the valve chattering or rapidly opening and closing, causing the internal parts to impact against each other. Typically, this is caused by oversized valves.
An unusual occurrence sometimes encountered is valve plugs or v-ports that have wear marks indicating a spinning action. This occurs in high-velocity gas lines, such as discharge lines or hot gas lines. The corrective action to eliminate this phenomenon is to reduce the gas velocity or install a valve with an anti-spin construction.
You may also encounter solenoid coils that fail, usually tripping the circuit breaker or blowing a fuse. This normally occurs due to the coil winding wire insulation breaking down internally and causing a short in the windings.
Other coil failures may be due to improper coil voltage (less than 85% or more than 110% of rated coil voltage), allowing the coil to be energized when not in place on the valve bonnet tube, or the plunger assembly not picking up when the valve is energized, causing the coil to overheat.
Control Arrangements, Control Valve SelectionSelecting control valves based on actual operating conditions is important. Changes in system loads will dictate whether one control valve or two control valves in parallel will be required to handle the system load.
Improperly sized valves can wear prematurely and fail at the most critical times. Dirt or contamination in the refrigeration system will affect reliable valve operation as well as other important system components. When-ever a system is serviced or a new system is installed, care should be taken to keep piping, vessels, evaporators, and other components as clean and contaminant-free as practical.
Another area of concern is pressure or temperature shock. This occurs when a sudden change in system pressure or temperature takes place and creates a severe strain on all system components. An example of such a condition occurs after termination of hot gas injection for low-temperature hot gas defrost evaporators.
In the past, systems were designed to open the suction stop valve after defrost and proceed directly into the refrigeration mode. When this occurred, there was generally about 70-psig pressure still in the evaporator which was then released directly into a 0-psig or less suction line, via a large-port automatic valve.
Many systems now incorporate a small bleed-down solenoid valve in parallel with the suction stop valve, which is controlled by a bleed-down period in the defrost cycle to open before opening the suction valve. This allows the evaporator to slowly equalize to suction pressure, thereby eliminating the sudden surge of pressure that occurs when a larger suction valve opens. Always consult the valve service bulletin or the factory before applying a valve in a unique manner or application.
As a final note, always be sure the control valve selected for an application is capable of performing the control function required of it. When questions arise, check the service bulletin, check with the factory, and recheck the application.
The initial time spent on this evaluation process may be insignificant compared to reviewing and correcting a misapplication of a control valve, once installed.
Warning: Failure, improper selection, or improper use of the products and/or systems described in this article can cause death, personal injury, and property damage. Due to the variety of operating conditions and applications for these products and systems, the user, through analysis and testing, is solely responsible for making the final selection of the products and systems, and for ensuring that all performance, safety, and warning requirements of the applications are met.
Information in this article was developed by the engineering and sales departments of the Parker Hannifin Refrigerating Specialties Division. For more information or detailed safety instructions, contact the company at 708-681-6300; 708-681-6306 (fax); www.parker.com/refspec (website). lN
Publication date: 02/12/2001