16.3 Solenoid Failure Modes, Strainers, King Valves, and Ammonia Piping

Solenoid valves, strainers, the king valve, and the piping code are the same chapter in the field: if dirt, a failed coil, or a trapped liquid line is in play, the operator who only “swaps the coil” will miss the real failure. CIRO treats these as Valves, Controls and DX plus the piping/materials knowledge that also shows up under safety standards.

Solenoid Failed Closed

A normally closed liquid-line solenoid is de-energized shut and opens when the coil is powered (room thermostat, PLC, defrost-not-active, pump-down permissive). Failed closed means the evaporator (or other user) does not get liquid when the control wants feed.

Typical causes:

• Burned or open coil, loose DIN connector, wrong voltage, blown fuse, interlock open
• Plunger stuck on rust, ice, or sludge; bent stem; missing manual-open stem left in the wrong position
• Plugged strainer or inlet orifice so the valve body never sees flow even if the plunger lifts
• Manual stem left front-seated closed after service

Plant signs: suction pressure falls on that circuit, superheat rises, the coil or process warms, frost stops at the solenoid and the outlet pipe goes from cold to ambient. On a water chiller, failed-closed feed plus continued pumping can freeze a barrel. Pump-down, if used, will look “successful” because no liquid is entering—then the circuit never refrigerates again. Do not confuse this with a failed-closed TXV; measure temperature drop across the strainer and solenoid first. A large drop there is restriction, not a bad power element.

Solenoid Failed Open

Failed open means liquid (or hot gas, if that is the valve) keeps passing after the coil is de-energized. Causes include a cut or wire-drawn seat, dirt holding the plunger off the seat, a plunger frozen in the lifted position, or a technician who left the manual stem in the open/service position.

Plant signs: the circuit will not pump down; suction stays up after the thermostat is satisfied; superheat collapses; liquid shows in the sight glass to the compressor or in the suction accumulator; oil foams. A hot-gas solenoid failed open puts the coil into a continuous partial defrost: high suction, poor refrigeration, frost melting while the room is calling for cooling. Failed-open liquid feed on a DX coil is a slugging and oil-dilution event. Isolate with the hand stop valve, not by arguing with the coil voltage. If the coil is de-energized and the pipe downstream is still liquid-cold, the valve is passing.

Solenoid Leaking

Leaking is the same failure as failed open, only slower. The seat weeps. Pump-down takes too long or never quite reaches the cut-out. A circuit that should be isolated during defrost still shows a wet suction. Over days, extra liquid in the low side looks like overcharge or a TXV that is “a little heavy.” Because the leak is small, coil voltage checks “prove” nothing. Use temperature, frost line, and whether the evaporator still feeds after the solenoid is commanded shut. Replace or rebuild the valve; do not keep opening the TXV to chase a wet coil.

Strainers Before TXVs and Pilots

Strainers (and inlet screens) belong immediately upstream of thermostatic expansion valves, electronic expansion valves, pilot-operated regulators, and solenoids. Their job is to catch mill scale, rust, PTFE tape, gasket crumbs, and weld beads before those particles hold a needle or pilot port off the seat.

A plugged strainer imitates a failed-closed solenoid or a starved TXV: high pressure drop, low capacity, hunting as the screen ices and thaws. Blow-down or service the strainer on the mechanical-integrity schedule, and always after construction or a compressor burnout. Pilot lines get their own small strainers; a dirty pilot makes a back-pressure regulator or hot-gas regulator hunt or stick. Never install a new TXV or pilot valve on a dirty line “just to see.”

King Valves, Isolation Valves, and Pump-Out

The king valve at the HPR liquid outlet isolates plant liquid feed. Isolation does not end there. Every major vessel, evaporator station, condenser, and compressor package needs stop valves so you can take a component out of service without pumping the entire charge into a truck.

Pump-out connections (and the procedures that use them) are how you empty a circuit before the second valve goes closed. Connect to a suction header, a dump station, or a designated pump-out point shown on the P&ID. Close the supply (often the king valve or the circuit liquid solenoid plus its stop), let the compressor or a transfer path pull the liquid out, then close the remaining stop. If liquid remains, you still need hydrostatic relief—a relief regulator to a lower-pressure vessel, a hydrostatic relief valve, or an administrative control that IIAR 2 actually allows for that configuration. “We will remember not to close both valves” is not a design for automatic solenoids that can trap liquid on a power loss.

King solenoids used for emergency liquid isolation must fail to the state the PHA requires, and they need the same strainer and seat discipline as any other solenoid. Closing the king valve in an emergency stops new liquid; it does not relieve trapped liquid in evaporators that are already full.

ASME B31.5 Ammonia Piping

ASME B31.5 (Refrigeration Piping and Heat Transfer Components) is the piping code IIAR 2 points to for ammonia refrigeration lines. It covers materials, design, fabrication, assembly, erection, examination, inspection, and testing of refrigerant and secondary-coolant piping. It does not replace the ASME Boiler and Pressure Vessel Code for the vessels; it starts at the first piping joint adjacent to those vessels, compressors, and pumps.

Materials. Anhydrous ammonia attacks copper and copper alloys and zinc. Galvanized coatings fail in ammonia service, especially with moisture. Do not use copper tube, brass fittings, galvanized steel, or zinc-coated parts in ammonia refrigerant piping. Compatible families are carbon steel, stainless steel, iron where the code and IIAR 2 allow the pressure/temperature, and aluminum in approved applications (evaporator coils, some equipment). Welding, threading, and flange facing follow B31.5 and the manufacturer. After any hot work, the mechanical-integrity program owns examination and leak test—not “it held overnight.”

Identification. Piping must be identifiable: ammonia (or R-717), function (liquid, suction, hot gas, relief, oil, thermosiphon), and pressure level (high side / low side). Operators and emergency responders should not have to guess which insulated line is liquid. Follow the plant’s IIAR 2-compliant labeling scheme and keep labels readable after insulation and ice.

Slope, traps, and supports.

• Slope high-pressure liquid and hot-gas lines so they drain toward vessels or intended drain points. Suction lines are designed for oil and vapor velocity; they still must not create a liquid trap that slugs a compressor on a restart.
• Traps (unintentional U-bends, a check plus a closed solenoid, a low point between two valves) hold liquid. Held liquid is a slug hazard and a hydrostatic hazard. Designed traps (oil pots, drain pots) have valves, heat, and relief on purpose.
• Supports must carry pipe weight plus ice. A suction line that sags into a new trap after a summer of missing hangers is a B31.5 support failure. Allow expansion; do not let a valve become the pipe anchor. Vibration at compressor discharge needs bracing so checks and weld-o-lets do not crack.

Dual Relief on Vessels

ANSI/IIAR 2 requires dual pressure-relief valves on pressure vessels with 10 cubic feet or more of internal gross volume, arranged on a three-way dual shut-off (transfer) valve so that one relief path is always open. You can isolate one PRV for replacement while the other remains in service. You cannot close both. Oil pots and some small separators may be under 10 ft³ and take a single PRV; almost every HPR, intercooler, recirculator, and large surge drum is above the threshold.

Relief inlet piping stays above the liquid level. Set pressure is at or below the vessel MAWP. IIAR 6 inspection, testing, and maintenance owns replacement or recertification intervals; industry practice discussed with IIAR 6 is commonly five years from installation for many ammonia PRVs, following the manufacturer and the current IIAR 6 program—do not treat a hallway rumor as a substitute for the plant’s MI procedure. Both valves on a dual assembly are in the time clock, not only the one that was aligned to the vessel.

Tie this back to the rest of the chapter: a king valve, a pair of circuit stops, and a tight check can create a liquid-full volume that the vessel PRVs never see. Hydrostatic protection is local to that trapped section. Dual relief on the HPR does not save a roof liquid line that was valved off in the sun.