Solenoid valves: Operation, selection, and application
Solenoid constructionThere are two types of solenoid valve construction: hermetic and take-apart.
Hermetic valves are brazed or welded together at the factory and have the advantage of being inherently free of external leaks.
Hermetic construction is most common in valves having a port size of 1/2 in. or less. Hermetic valves, of course, cannot be disassembled for cleaning or repair.
Take-apart types can be serviced and repair kits are normally available from the manufacturer. Some form of seal, usually elastomer gaskets or O-rings, is required.
This produces the potential for leakage after a period of service, particularly when prolonged high temperatures are encountered. The variety of refrigerants and lubricants now used results in a difficult application for seal materials.
Also, formulas published by O-ring manufacturers indicate that permeability directly through the seal material can account for as much as 0.25 oz/year of refrigerant loss in a medium- to large-size valve.
Piloted or directly operatedValves with a small port, or those that have to operate at only a limited pressure differential, are usually actuated directly by the solenoid plunger. The magnetic pull generated by the coil windings is used to operate the main valve pin, or poppet, directly.
A directly operated solenoid is illustrated in Figure 1.
The force required is simply the port area multiplied by the maximum operating pressure differential. For refrigeration service, a pressure differential of at least 300 psi is usually needed. This limits the port diameter to about 1/8 in.
A trick sometimes used to increase the maximum pressure differential is the addition of free play, or travel, between the plunger and valve pin. This permits the plunger to start in flight and gain momentum before attempting to lift the valve pin from the port.
This is referred to as a “hammer blow” design.
Pilot-operated valves can be made with very large main ports, because the magnetic pull is used only to operate a small pilot port. Figure 2 shows a pilot-operated valve with a diaphragm-type main valve element.
The flexure of the diaphragm allows the main poppet to lift from the main port. The diaphragm has a small hole located so that valve inlet pressure is bled to the top of the diaphragm.
As long as the pilot valve is closed, inlet pressure is exerted on the top side of the diaphragm, and the main poppet is held tightly on the valve seat by the pressure differential between inlet and outlet.
When the pilot valve is opened, pressure above the diaphragm is exhausted to the valve outlet, and inlet pressure on the underside of the diaphragm causes the main valve to lift from the port. Since there is usually a return spring urging the main valve closed, a minimum pressure differential between valve inlet and outlet may be required to maintain the valve fully open against the spring load.
Most pilot-operated valves require minimum pressure drop to provide stable, full-open operation. Special models are sometimes offered for “zero-differential” pressure. These may use an external pressure source as the actuating power.
Some designs use a piston instead of a diaphragm. The piston has a small bleed hole, and operation is the same as a diaphragm valve. When the pilot exhausts pressure from above the piston, inlet pressure causes it to lift, opening the main port. Figure 3 shows a piston valve.
Figure 4 (page 28) shows a slightly different pilot arrangement. In this case, the pilot valve is incorporated into the piston. The pilot port goes directly through the piston to the outlet passage.
This very simple arrangement is often used on valves of 7/16-in. port size or less.
There is a basic size limitation for this design, since the pilot valve plunger must travel slightly farther than the piston stroke. If the plunger stroke is too long, the magnetic pull is greatly reduced.
The focus of the foregoing has been on two-way valves but, of course, other varieties of solenoid valves are available and the same basic principles apply. These other varieties include three- and four-way and normally open valves.
The larger sizes use a pilot valve to generate a pressure unbalance, causing the main valve to lift or slide.
Valve selectionValves should not be selected by line size. The maximum allowable pressure drop should be determined from system requirements.
For example, a low-temperature refrigeration system may not tolerate more than a 0.25-psid pressure drop in a suction line solenoid. A hot gas defrost valve for the same system would probably be sized to pass the requisite flow at a pressure drop of near high- to low-side differential — perhaps 150 psid.
Published capacity data should always be used for size selection. It is important to avoid oversizing a piloted valve such that the minimum allowable operating pressure drop does not develop. This may cause the valve to chatter, “chug,” or fail to open fully.
If there is a minimum operating pressure differential limit, it will be published along with other valve data.
It is also important not to exceed the maximum operating pressure differential (MOPD) between inlet and outlet. By safety agency requirements, solenoids must be able to operate at their MOPD rating at 85% of rated line voltage. The same valve typically has a lower MOPD when a direct current coil is used.
Valves must also have a maximum rated pressure (MRP) of at least the maximum pressure expected on the application. The MRP is the maximum internal pressure to which the valve may be subjected as specified by the manufacturer.
Use valves only with the fluids they were designed to handle, since internal materials may be incompatible with other fluids. Also, do not use a general-purpose valve for shutting off of combustibles, such as natural gas or propane. These applications require a solenoid valve that has met special safety agency requirements for “safety shut-off service.”
Coil housingsMost solenoids are available with a variety of coil housing types. The proper type depends on the exposure to the elements, and the degree of protection afforded by the enclosure in which the valve is installed.
Coil housing types are designated by ANSI/NEMA (American National Standards Institute/National Electrical Manufacturers Association) standards as suitable for certain applications. In general, the higher the ANSI/NEMA type number, the greater the exposure to the elements that can be permitted.
For example, NEMA Type 1 is suitable for most indoor applications, while NEMA Type 4 is suitable for most outdoor applications. For locations where there are explosive vapors or dust, special explosion-proof housings, designated NEMA Type 7 or Type 9, are required. Usually, both conduit and junction box styles are available, as well as European DIN styles.
A complete treatment of ANSI/NEMA and safety agency standards is well beyond the scope of this article. For help in selecting the appropriate coil housing for the exposure expected, the manufacturer’s literature or application engineering department should be consulted.
Electrical ratings and performanceIn addition to voltage and frequency, specifications include holding current and inrush current as well as wattage.
Inrush is the current that prevails immediately after the switch is closed, but before the plunger has completed its travel.
The current draw of ac coils is determined by coil impedance, not resistance alone. Impedance results from a combination of resistance and inductive reactance.
Inductive reactance is a measure of how much the flow of current is inhibited as a result of the rapidly reversing magnetic fields associated with an alternating current. Inductive reactance increases both with frequency and with the amount of iron in the core of the solenoid. This is why current draw is higher at 50 Hz than at 60 Hz. It also explains why inrush current is often twice as high as holding current.
When the switch is first closed, the plunger is at some distance from the stationary pole, so that an air gap greatly limits the magnetic field strength. After the plunger reaches its final position, the air gap is closed and the effectiveness of the iron in the core is much greater.
This causes the inductive reactance, and therefore the impedance, to be much higher after the plunger completes its flight and solidly contacts the stationary pole.
Current in a dc solenoid depends only on resistance, so inrush and holding current are no different. This is why the MOPD rating is lower with direct current; there is no benefit of initial over-current to assist in lifting the plunger.
Recently, electronically enhanced dc coils have been developed which artificially produce an over-current for the first few milliseconds, simulating the inrush current of ac and providing full MOPD.
InstallationThe instruction sheet that comes with the valve is the best authority on installation, but here are a few points worth emphasizing.
- Â Take-apart valves with extended copper tubes are designed to be brazed without disassembly, with proper wet-ragging procedures. This is preferred, since valves are leak tested at the factory and the possibility of a leak developing during field reassembly is eliminated. Also, no parts can be lost or misassembled.
- Â If a valve is equipped with a manual-opening stem, be sure to open it during a nitrogen brazing purge and subsequent evacuation. Also, in some designs, having an elastomer or plastic seat material out of contact with the hot valve body during brazing provides an added margin of safety from heat damage.
- Â For an ac coil that is not installed on a valve, never apply power for more than a few seconds. As noted earlier, current draw is greatly influenced by the amount of iron in the solenoid core. When the coil is uninstalled, there is no iron and current can be as much as four times’ normal.
- Â It is recommended that valves not be installed with the plunger enclosing tube (coil tube) angled down below the horizontal. The reason is that system debris can collect in the tube, preventing proper magnetic contact between the plunger and the stationary pole piece in the top. This can cause coil buzz and excessive current draw.
TroubleshootingAgain, the valve instruction sheet usually gives troubleshooting tips, but here are some causes of trouble that may be more elusive.
- Â Under-voltage as well as over-voltage can cause coil burnout, the same as a motor. Also, pressure differentials that are too high for the valve to open against can cause coil burnout, because current draw tends to remain near the inrush value.
- Â An oversized solenoid valve of the piloted type may behave strangely at some operating conditions, where there is insufficient pressure differential to open it or maintain it in a stable open condition. In these cases the valve may chug or chatter, or may seem to open just partially, producing a very low flow.
- Â Make sure that the design pressure drop is always at least equal to the minimum differential that the valve is rated for.