Monday, April 23, 2018

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Discrete Control Elements - Solenoid Valve Actuators

A very common form of on/off valve used for pneumatic and hydraulic systems alike is the solenoid valve. A “solenoid” is nothing more than a coil of wire designed to produce a magnetic field when energized. Solenoid actuators work by attracting a movable iron armature into the center of the solenoid coil when energized, the force of this attraction working to actuate a small valve mechanism. Solenoid-actuated valves are usually classified according to the number of ports (“ways”). A simple on/off solenoid valve controlling flow into one port and out of another port is called a 2-way valve. Another style of solenoid valve, where flow is directed in one path or to another path – much like a single-pole double-throw (SPDT) electrical switch – is called a 3-way valve because it has three fluid ports.

2-way solenoid valves

Solenoid valve symbols often appear identical to fluid power valve symbols, with “boxes” representing flow paths and directions between ports in each of the valve’s states. Like electrical switches, these valve symbols are always drawn in their “normal” (de-energized) state, where the return spring’s action determines the valve position:


2 Way Solenoid Valves in energized and not energized state


Unlike electrical switches, of course, the terms open and closed have opposite meanings for valves. An “open” electrical switch constitutes a break in the circuit, ensuring no current; an “open” valve, by contrast, freely allows fluid flow through it. A “closed” electrical switch has continuity, allowing current through it; a “closed” valve, on the other hand, shuts off fluid flow.

The arrow inside a solenoid valve symbol actually denotes a preferred direction of flow. Most solenoid valves use a “globe” or “poppet” style of valve element, where a metal plug covers up a hole (called the “seat”). Process fluid pressure should be applied to the valve in such a way that the pressure difference tends to hold the solenoid valve in its “normal” position (the same position as driven by the return spring). Otherwise1, enough fluid pressure might override the return spring’s action, preventing the valve from achieving its “normal” state when de-energized. Thus, we see that the label “2-way” does not refer to two directions of flow as one might assume, but rather two ports on the valve for fluid to travel through.

Some solenoid valves are designed in such a way that the direction of fluid flow through them is irrelevant. In such valves, the arrow symbols will be double-headed (one head at each end, pointing in opposite directions) to show the possibility of flow in either direction.

3-way solenoid valves

3-way solenoid valves have three ports for fluid, with two positions customarily referred to as normally-open and normally-closed. Ports on a 3-way valve are commonly labeled with the letters “P,” “E,” and “C,” representing Pressure (compressed air supply), Exhaust (vent to atmosphere), and Cylinder (the actuating mechanism), respectively. Alternatively, you may see the cylinder port labeled “A” (for actuator) instead of “E”.


3 way solenoid valves in energized and not energized state

Alternatively, the numbers 1, 2, and 3 may be used to label the same ports. However, the numbers do not consistently refer to pressure source (P) and exhaust (E) ports, but rather to the 3-way valve’s “normal” versus “actuated” statuses. A 3-way valve will pass fluid between ports 1 and 3 in its “normal” (de-energized) state, and pass fluid between ports 1 and 2 in its energized state. The following table shows the correspondence between port numbers and port letters for both styles of 3-way solenoid valve:

Valve type
Pressure (P) port
Exhaust (E) port Cylinder (C) port
 Normally-closed 2 3 1
 Normally-open 3 2 1


As with 2-way solenoid valves, the arrows denote preferred direction of fluid flow. Bidirectional 3-way valves will be drawn with double-headed arrows (pointing both directions). A different symbology is used in loop diagrams and P&IDs than that found in fluid power diagrams – one more resembling general valve symbols in instrumentation:


symbology used in loop diagrams and P&IDs

Unfortunately, these symbols are not nearly as descriptive as those used in fluid power diagrams. In order to show directions of flow (especially for 3-way valves), one must add arrows showing “normal” (de-energized, DE) flow directions:


symbology used in loop diagrams and P&IDs at normal or de-energized state

Alternatively, a pair of arrows shows the directions of flow in both energized (E) and de-energized (D) states:


symbology used in loop diagrams and P&IDs in energized and de-energized states


Photographs of an actual 3-way solenoid valve (this one manufactured by ASCO) appear here:


ASCO 3 way solenoid valve


A view of the nameplate for this particular solenoid valve reveals some of its ratings and characteristics:


ASCO Solenoid Valve Nameplate


4-way solenoid valves

When a pneumatic actuator requires air pressure applied to two different ports in order to move two different directions (such as the case for cylinders lacking a return spring), the solenoid valve supplying air to that actuator must have four ports: one for air supply (P), one for exhaust (E), and two for the cylinder ports (typically labeled A and B). The following diagram shows a 4-way solenoid valve connected to the piston actuator of a larger (process) ball valve:


4 way solenoid valve diagram



The same diagram could be drawn using the “triangle” solenoid valve symbols rather than the “block” symbols more common to fluid power diagrams:


Triangle Solenoid Valve Diagram

Here, the letters “D” and “E” specify which directions air is allowed to flow when the solenoid is de-energized and energized, respectively.


Normal energization states

Solenoid valves may be used in such a way that they spend most of their time de-energized, energizing only for brief periods of time when some special function is required. Alternatively, solenoids may be maintained in an energized state, and de-energized to perform their design function. The choice to use a solenoid’s energized or de-energized state to perform a specific function is left to the system designer, but nevertheless it is important for all maintenance personnel to know in order to perform work on a solenoid-controlled system.

Take the following segment of a P&ID for a steam turbine-driven pump control system for example, where a pair of 3-way solenoid valves control instrument air pressure to a piston-actuated steam valve to start the turbine in the event that an electric motor-driven pump happens to fail:


Steam Turbine Driven Pump Diagram

If either of the two solenoid valves de-energizes, instrument air pressure will vent from the top of the piston actuator to atmosphere, causing the steam valve to “fail” to the full-open position and send steam to the turbine. This much is evident from the curved arrows showing air flowing to the “Vent” ports in a de-energized (DE) condition. An additional valve (PY-590) ensures the piston actuator’s motion by simultaneously applying air pressure to the bottom of the actuator if ever air is vented from the top. As an additional feature, the left-hand solenoid valve (SOV-590A) has a manual “Reset” lever on it, symbolized by the letter “R” inside a diamond outline.

The only indication of the solenoids’ typical status (energized or de-energized) comes from the letters “NE” next to each solenoid coil. In this case, “NE” stands for normally energized. Therefore, this steam turbine control system, which serves as a back-up to an electric motor-driven pump, relies on either (or both) of the solenoid valves de-energizing to make the turbine start up. Under “normal” conditions, where the turbine is not needed, the solenoids remain energized and the steam valve remains shut.

Unfortunately, this use of the word “normal” is altogether different from the use of the word “normal” when describing a solenoid valve’s open/close characteristics. Recall that a normally open solenoid valve allows fluid to pass through when it is de-energized. A normally closed solenoid valve, by contrast, shuts off fluid flow when de-energized. In this context, the word “normally” refers to the unpowered state of the solenoid valve. This is quite similar to how the word “normally’ is used to describe switch contact status: a normally-open (NO) electrical switch is open when unactuated; a normally-closed (NC) electrical switch is closed when unactuated. In both cases, with solenoid valves and with electrical switches, the word “normally” refers to the condition of minimum stimulus.

However, when an engineer designs a solenoid control system and declares a solenoid to be “normally energized,” that engineer is describing the typical status of the solenoid valve as it is intended to function in the process. This may or may not correspond to the manufacturer’s definition of “normally,” since the solenoid manufacturer cannot possibly know which state any of their customers intends to have their solenoid valve typically operate in. To illustrate using the previous steam turbine control system P&ID, those two solenoid valves would be considered normally closed by the manufacturer, since their de-energized states block air flow from the “P” port to the “C” port and vent air pressure from the “C” port to the “E” (vent) port. However, the engineer who designed this system wanted both solenoids to be energized whenever the turbine was not needed to run (the “normal” state of the process), and so the engineer labeled both solenoid coils as normally energized, which means both solenoids will be actuated to pass air pressure from their “P” ports to their “C” ports (and close off the vent ports) under typical conditions. Here, we see the manufacturer’s definition of “normal” is precisely opposite that of the process engineer’s definition of “normal” for this application.

The manufacturer’s and process engineer’s definitions of “normally” are not always in conflict. Take for example this P&ID segment, showing the solenoid control of an air vent door on a large furnace, designed to open up if the forced-draft fan (blowing combustion air into the furnace) happens to stop for any reason:

Solenoid Controlled Air Vent Door

Here we have a normally open solenoid valve, designed by the manufacturer to pass instrument air pressure from the pressure (“P”) port to the cylinder (“C”) port when de-energized. The straight arrow with the “DE” label next to it reveals this to be the case. Instrument air pressure sent to the air door actuator holds the door shut, meaning the air door will swing open if ever instrument air pressure is vented by the solenoid. For this particular solenoid, this would require an energized condition.

The process engineer designing this emergency Air Door control system choose to operate the solenoid in its de-energized state under typical operating conditions (when the furnace air door should be shut), a fact revealed by the letters “NDE” (normally de-energized) next to the solenoid coil symbol. Therefore, the “normal” process operating condition for this solenoid happens to be de-energized, which makes the manufacturer’s definition of “normal” match the engineer’s definition of “normal.” The solenoid valve should be open (passing air to the door’s actuating cylinder) under “normal” operating conditions.

1One could argue that enough fluid pressure could override the solenoid’s energized state as well, so why choose to have the fluid pressure act in the direction of helping the return spring? The answer to this (very good) question is that the solenoid’s energized force exceeds that of the return spring. This is immediately obvious on first inspection, as the solenoid must be stronger than the return spring or else the solenoid valve would never actuate! Realizing this, now, we see that the spring is the weaker of the two forces, and thus it makes perfect sense why we should use the valve in such a way that the process pressure helps the spring: the solenoid’s force has the best chance of overcoming the force on the plug produced by process pressure, so those two forces should be placed in opposition, while the return spring’s force should work with (not against) the process pressure.


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