16.2 Check Valves, Discharge Checks, and Defrost Check Valves

Check valves look simple: flow one way, block the other. On an industrial ammonia plant they are safety devices. A missing, reversed, chattering, or leaking check can reverse a screw compressor, flood a suction header during defrost, or put hot gas into a coil that is supposed to be feeding. CIRO expects you to know where the check belongs, why it is there, and what chatter and leakage do to the plant.

What a Check Valve Is Doing

A check valve (non-return valve) opens when upstream pressure exceeds downstream pressure by the spring or disc cracking pressure, and it seats when flow tries to reverse. Types you will see on ammonia piping include swing checks, lift/piston checks, in-line spring checks, and stop-check combinations. Swing checks are common on larger lines; piston or spring-assisted “non-slam” checks are common on screw discharge lines where flow starts and stops quickly.

The body has a flow arrow. That arrow is the only legal installation direction. A check installed backward is a closed valve until someone cuts it out. A check installed in the wrong plane (a swing check that needs horizontal flow standing in a vertical riser, or the reverse) will flutter, hang open, or never seat.

Unlike a solenoid or a stop valve, a check is not a reliable isolation device for pump-out. Seats leak. Treat a check as flow direction control, then use stop valves and pump-out connections when you need a gas-free isolation.

Discharge Check Valves

Reciprocating compressors have suction and discharge valves on the valve plate. They still often get an external discharge check, especially when several recips share a discharge header. Screw and rotary-vane compressors do not have those built-in discharge valves. IIAR basic training is explicit: rotary and screw machines must have a discharge-line check valve so high-pressure discharge vapor cannot flow back into a stopped compressor.

Reverse flow into a stopped screw is ugly in several ways at once:

• High-pressure gas from the header can spin the rotors backward. Screws are not designed to be turbines. Reverse rotation can damage rotors, bearings, and the shaft seal, and it can drive the motor backward if the coupling is still engaged.
• Oil in the separator and discharge piping can be pushed back into the compressor and then into suction, emptying the oil system and flooding the low side with lubricant.
• The suction side of that machine can be driven up toward discharge pressure, lifting reliefs, opening internal safety valves, or forcing gas into a shared suction header.

On parallel machines, the running compressor’s discharge is a ready source of reverse flow into any stopped unit that lacks a tight discharge check. This is why each compressor package gets its own discharge check, usually close to the package, often downstream of the oil separator, with a stop valve for isolation. Some packages add a suction check as well; the discharge check is the one that protects against header reverse flow.

When a compressor unloads or trips, discharge flow falls. A heavy swing disc can slam or chatter on residual pulsation. Spring-assisted piston checks are chosen to close quickly without hammer. If you hear a discharge check hammering on every capacity step, the valve may be oversized (not enough flow to hold the disc steadily open), the spring may be wrong, or a recip’s pulsation is beating a swing check that should have been a non-slam design.

Defrost Check Valves

Hot-gas defrost takes high-pressure discharge gas into an evaporator to melt frost. During that sequence the coil is no longer a low-pressure evaporator; it is a temporary condenser. Liquid and gas will go wherever pressure differences allow unless check valves and solenoid sequencing force the right path.

Defrost check valves (and related hot-gas / drain-line checks) exist to stop the wrong path:

• Keep hot gas from backing into a suction header, a neighboring evaporator that is still in refrigeration, or a hot-gas supply that is supposed to be one-way into this coil.
• Keep condensed liquid that forms in the defrosting coil from draining backward into the hot-gas header, into another coil’s feed, or into a suction line that cannot handle a slug.
• Hold liquid in the drain pan circuit or defrost regulator path so the coil actually warms instead of dumping its heat into the wrong vessel.

A typical evaporator station has a liquid solenoid (and often a TXV or hand expansion), a suction stop or regulator, a hot-gas solenoid, and checks on the hot-gas inlet and sometimes on the liquid/suction connections so pressure during defrost cannot reverse into the liquid header. Soft-gas or two-step hot-gas sequences still rely on those checks. If a defrost check is stuck open, you will see suction pressure rise on circuits that should be in refrigeration, frost melting on the wrong coil, or hot-gas disappearing into the suction header. If the check is stuck closed or installed backward, that coil will not defrost: no hot gas in, or condensate cannot leave on the intended drain path.

Defrost checks see wide temperature swings—cold liquid one hour, hot discharge gas the next. Seats and springs fatigue. Oil and ice can hold a disc off the seat. After a failed defrost, do not assume the solenoid is the only suspect; include the check in the isolation and inspection.

Installation Direction, Chatter, and Leakage

Direction. Match the arrow to intended forward flow. On a compressor discharge, forward flow is compressor-to-header. On a hot-gas branch, forward flow is header-to-evaporator (unless the P&ID shows a different condensate drain path). Vertical versus horizontal is not optional: follow the manufacturer. Piston checks often need a minimum pressure drop to stay open; an oversized valve on a lightly loaded machine will flutter.

Chatter is the disc opening and slamming many times per minute. Causes include:

• Reciprocating pulsation on a swing check that should be damped or replaced with a spring check
• A check oversized for current mass flow (slide valve unloaded, VFD at low speed, one of several parallel machines running)
• A check undersized or a disc that never lifts fully, so it sits in the flow and vibrates
• Missing or failed damping spring
• Installation too close to an elbow or a stop valve that sheds turbulence onto the disc

Chatter is not just noise. It beats the seat, work-hardens the disc, fatigues welds, and is the usual path to leakage.

Leakage shows up as a stopped compressor whose discharge temperature or suction pressure creeps toward the header, oil level that drops after a trip, or a defrost that will not isolate. A leaking discharge check on a parallel screw is an urgent mechanical-integrity item: the next start can be against reverse rotation or a dry oil sump. A leaking defrost check puts extra load on the suction, ruins superheat control, and can slug a compressor after defrost if liquid has been sitting in the wrong pipe.

Operator Checks

• Confirm the flow arrow before you sign a work order that “just replaced the same valve.”
• After a compressor trip, watch the stopped machine’s suction and oil level. Rising suction or falling oil with a sister machine running points at discharge-check leakage.
• During hot-gas defrost, neighboring coils should stay in refrigeration temperatures. A neighboring suction that warms in step with someone else’s defrost is a check or solenoid passing hot gas the wrong way.
• Do not use a check as the only isolation for welding or opening a line. Seat leakage is expected over life. Add stop valves, pump out, and verify pressure.
• After installing a new check, include it in the hydrostatic/trapped-liquid review: a check that holds can trap liquid between itself and a closed solenoid just as a second stop valve would.