Take it to the limit
One more time

("Take It To The Limit," by The Eagles, 1975)

PRESSURE SWITCHES 

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ... already learned about the following "Dry" Pressure Sensors in PTOA Segment #227.

• The Diaphragm and Capsule Diaphragm.
• The Bellows and Stacked Bellows.
• The C-Shaped, Spiral-Shaped, and Helix-Shaped Bourdon Tube.

 

These Pressure Sensors can be incorporated into "On-Off Control" devices known as electromechanical Pressure Switches. The descriptor "electromechanical" simply means that these switches are not 100% electrical Pressure Switches. Electromechanical Pressure Switches rely on the mechanics of the "Dry" Pressure Sensor to sense a change in the PV Pressure.  

The most common electromechanical Pressure Switches are composed of two functioning parts:

• A Pressure Sensor that interfaces with a
• Switching Element that contains Contacts and Terminals.

 

The Pressure Sensor is more than likely one of the Pressure Sensors listed above, however other models do exist. For example, a Diaphragm has been incorporated into the Pressure Switch that appears on the left-hand side of the nearby gif. The right-hand side of the gif features a Piston-type Pressure Sensor incorporated within the Pressure Switch. 

No matter what type of Pressure Sensor is chosen for the Pressure Switch, they all work the same:

The Pressure Sensor moves in response to the sensed changes in the PV Pressure.

 

This movement of the Pressure Sensor impacts the closing or opening of the Switching Element.

 

FUNCTION OF PRESSURE SWITCHES

Pressure Switches can be used for "On-Off Control" or "Off-On Control."

The "bench state" or "shelf state" of the Pressure Switch is the pre-set state of the Pressure Switch when the device arrives from the factory.

The bench state/shelf state of a Pressure Switch is either Normally Open (NO) or Normal Closed (NC). The desired control scheme outcome will determine whether the bench state/shelf state of the Pressure Switch is Normally Closed (NC) or Normally Open (NO).

The Instrument Tech and Process Operator must be aware of the bench state/shelf state of each Pressure Switch because the bench state/shelf state of the device will also be the "Fail-Safe Position" of the Pressure Switch..  The "Fail-Safe Position" of the Pressure Switch is the state of the switch upon loss of electrical power.

Once the sensed PV Pressure attains the pre-set Set Point Pressure of the Pressure Switch, the Switching Element is engaged and the Normally Open (NO) Pressure Switch changes to Normally Closed (NC), or vice versa.

Oops! Fred is confused again.

Fred, look at the nearby gif of a simple electrical circuit.

Assume the "normal condition" of the light is when it is "Off." Otherwise stated, the light in the circuit is not normally on.

Note that the little red arrow pointing to the gray switch box is on the left-hand side of the box when the light is "Off." The light is "Off" in the Normally Open position of the switch because the circuit is not complete.

When conditions change, the little red arrow moves to the right side of the gray switch box, which is the Normally Closed (NC) position. The circuit is completed, and the light turns "On."

Note that the light in the above simple electrical circuit is either 100% "On" or 100% "Off."  The light cannot be "sort-of On" or "sort-of Off."  The ten-dollar phrase for "On-Off Control" is "Discrete Control."

"Discrete Control" has nothing to do with being secretive. The label seems to be applied to mechanical control devices ... like Pressure Switches with mechanical Pressure Sensors. The increasing use of computers in process control is increasing the use of "Binary Control." Hence the use of a "0"(zero) or "1" to change from one state/condition to a different state/condition is the  modern version of Discrete Control.

Now assume that a mechanical Pressure Switch is what causes the little red arrow to move from the Normally Open to the Normally Closed position, thus turning the light "On."

Otherwise stated, imagine that an increase in the PV Pressure sensed by a Pressure Sensor is what makes the little red arrow move from the NO state on the left to the NC state on the right once the Set Point Pressure is attained.

Next, imagine a subsequent decrease in sensed PV Pressure makes the little red arrow move from the right NC state back to the left NO state, hence the light turns off again.

Voila! The above imagined Pressure Switch connected to the light circuit gif illustrates how a Pressure Switch can be used for Discrete Control. Depending upon how the control scheme is designed, a 100% Normally Open state will become a 100% Normally Closed state (or vice versa) once the Pressure Sensor attains the Set Point Pressure.

 

Common Pressure Switch Example: The Dashboard "Check Oil" Lamp

A common example of a working Pressure Switch doing its job is the "Check Oil" lamp indicator on an automobile's dashboard. The light will come on when oil pressure in the reservoir drops to the Set Point Low-Oil Pressure.

The "Check Oil" Lamp will come on the dashboard when the following occurs:

• The labelled red Contact Point of the Low-Oil Pressure Switch is normally held aloft of the (unlabeled) red Contact Point circuitry base by the blue Springs. Hey, Fred! That statement means the control circuit is Normally Open while the Oil Pressure is above the Low-Pressure Set Point.

• Once the level of the oil decreases, so does the PV Pressure sensed on the underside of the pressure-sensing black Diaphragm.
• As the Oil Pressure decreases to a preset Low-Pressure Set Point, the Contact Point is extended by the blue Springs until contact is made with the red Contact Point Circuitry Base. The PV Pressure sensed when this contact is made is the Low-Pressure Set Point of the Pressure Switch. 
• The contact between the red Contact Point and the red Contact Point circuit base completes the circuit. Otherwise stated: the state of the electrical circuit changes from Normally Open to Normally Closed.  The Indicator Lamp on the dashboard lights up since the circuit us closed.
• Assume the automobile owner replenishes the oil in the oil reservoir.  The oil Pressure sensed by the black Diaphragm will increase. The increase in PV Pressure will lift up the black Diaphragm. The red Contact Point will be pushed up and off of the red Contact Point Circuitry base once the oil pressure exceeds the Low-Pressure Set Point Pressure.  The circuit will return to the Normally Open state and the light on the dashboard will go out.

 

HOW TO IDENTIFY A PRESSURE SWITCH ON A P&ID

The ISA Symbol for a Pressure Switch is shown on the nearby graphic. A Low-Pressure Pressure Switch is labelled PSL. A High-Pressure Pressure Switch is labelled PSH.

Th nearby schematic shows how PSL and PSH are used to monitor the PV Pressure in the top levels of a vessel.  The output from the PSL and PSH are both sent to a common diamond-shaped control decision point.

The PSL and PSH as shown are not Differential Pressure Switches. However, Differentials Pressure Switches are commonly used in industry and incorporate Differential Pressure Sensing Pressure Sensors that were featured in PTOA Segment #227.

The Pressure Sensor in a Differential Pressure Switch senses the High Pressure on one side and the Low Pressure on the other side. As long as the Pressure is balanced there is no movement of the Pressure Sensor. When the PV Pressure on the High-Pressure Side of the Pressure Sensor increases ... or the PV Pressure on the Low-Pressure Side of the Pressure Sensor decreases, the Pressure Sensor moves. The movement of the Pressure Sensor causes the Switching Element to change from NO state to the NC state ... or vice versa.

Notice the Pressure Switch on the left side of the vessel in the nearby graphic that is labelled PSHH. This Pressure Switch is triggered by a very high (high-high) Set Point Pressure.

The output from the electrical circuit of this (Normally Open) PSHH Pressure Switch causes a local High-Pressure Alarm (PAH) to blare.

Pressure Switches are also used for limiting increases in the PV Pressure. Brilliant PTOA Readers and Students learned about Steam Turbines in PTOA Segment #193. The PV Pressure of the feedstock steam that flows into the Steam Turbine must not exceed design limits. When the PV Pressure of the steam exceeds the limit, a Pressure Switch can divert higher Pressure steam to a Pressure Safety Valve (PSV) that vents the steam.

 

THE POWER OF DISCRETE CONTROL

At first glance, the power of simple Discrete Control (aka "On-Off Control") might be underestimated.

Discrete Control Loops can do much more than turn on lights or sound alarms. The output from a Pressure Switch can be fed to a powerful Programmable Logic Controller (PLC).  PLCs compare the output from multiple Pressure Switches and then decide (in much less time than a human being can decide) what action to take based on the multiple inputs.

In the nearby graphic, a Temperature Switch and a Pressure Switch are monitored by an Allen Bradley PLC. A Level Switch is activated by a float in an unseen vessel. The status of each switch and its respective tripping point (Set Point) are shown below:

 

 

 

FORM OF PRESSURE SWITCHES

Starting from the bottom to the top, the critical hardware in a Pressure Switch is labelled in the nearby schematic:

• A Port where the PV Process Pressure interfaces with the ...
• Pressure Sensor (Diaphragm, Bellows, Bourdon Tube etc).
• A Pressure-Range Spring that counteracts the sensed PV Pressure and which WILL NOT MOVE UNTIL THE PV PRESSURE EXCEEDS SET POINT.
• A Pressure Sensor Assembly that protects the Pressure Sensor from the Process Stream (labelled Piston Assembly in the graphic).
• A Set Point Adjusting Nut (could be a screw) that impacts the tension of the Pressure-Range Spring.
• A Pressure Switch Element that includes Contacts and Terminals labelled:  Normally Closed = NC, Normally Open = NO contact Common = C.

 

Focus on the Pressure Switch Contacts and Terminals: NC, NO, and C

The mechanical part of an electromechanical Pressure Switch is everything except the Switching Element which contains the Contacts and Terminals.

The Pressure Switch has 3 Contact Terminals; however only two of the 3 Contacts are typically connected to wire.

The Common "C" Terminal supplies the power for the circuitry and is always connected.

The other wire is either connected to the Normally Closed (NC) or Normally Open (NO) Terminal.

As was stated above, the "bench-state/shelf-state" of the Pressure Switch will determine whether the Pressure Switch is Normally Open or Normally Closed and will also be the Fail-Safe Position of the Pressure Switch.

 

 

DEADBAND

Discrete Control would drive any Process Operator or Instrument Tech nuts without Deadband.

Deadband is the difference between the Set Point and the Reset Point of a Switch. The amount of Deadband desired is specified when the Switch is ordered. 

In the nearby graphic, the two-headed black arrow points to the difference between the Set Point and the Reset Point of what could be a Pressure Switch, a Temperature Switch, a Flow Switch, or a Level Switch.

In a Pressure Switch, the Pressure is sensed by a Pressure Sensor. 

 

Assume the graph illustrates the activity of a Normally Closed (NO) High Pressure Switch (PSH). The X-axis is Time and the Y-Axis is Pressure in units of psi.

The "beginning of time" is on the far-left side of the graph. This graph shows how the process PV Pressure sensed by the Pressure Sensor in the Pressure Switch changes over time. Note how the Pressure cycles above the Set Point, below the Reset Point and then begins to repeat the cycle.

Assume the  top arrowhead of the Deadband Arrow is the Set Point Pressure of 1200 psi. Assume the bottom arrowhead of the Deadband Arrow is the Reset Point Pressure of 1150 psi.

The PSH as described has a Deadband of 50 psi. This Deadband was specified by the customer when the Pressure Switch was ordered. Specifying the Set Point Pressure and Deadband desired determines the Reset Point Pressure.

Note how from "the beginning of time" on the leftmost side of the graph, the PV Pressure that is sensed by the Pressure Sensor increases to the Set Point Pressure (1200 psi) of the Normally Open PSH. Upon attaining the Set Point Pressure, the switch is tripped to the Normally Closed state.

The triggered Pressure Switch ... now in the NC state ...  initiates other process actions to occur. Eventually a decrease in the PV Pressure of the system is sensed by the Pressure Sensor in the Pressure Switch, albeit not until after the sensed PV Pressure has surpassed the Set Point Pressure.

After peaking above Set Point Pressure, the sensed PV Pressure starts to decrease.

The PV Pressure sensed by the PSH's Pressure Sensor eventually declines below the Set Point Pressure of 1200 psi and continues to decrease to the Reset Point Pressure of 1150 psi

Upon decreasing to the Reset Point Pressure, the Normally Closed position of the Pressure Switch changes back to a Normally Open position.

The triggered Pressure Switch ... once again returned to the NO state ... alerts other process actions to take place which eventually stop the decrease in PV Pressure, albeit not before the sensed PV Pressure valleys below the Reset Point Pressure.

Voila! The above paragraphs explain why the natural outcome of Discrete Control with a PSH can be visualized as a continuous wave cycling above Set Pont Pressure and below Reset Point Pressure.

The increase in Pressure above Set Point Pressure and below Reset Point Pressure happens because the mechanics of the control scheme cannot work instantaneously. Otherwise stated, it takes time for the change in Pressure caused by the Switch Element to be sensed and acted upon by the system that is having its PV Pressure controlled. Process Operators must remember that Deadband will result in time delay between the tripping of a Pressure Switch and observing the outcome of the switch having been tripped.

Beware! If the optimal Pressure Sensor is not selected for the optimal service of a Low Pressure Switch (PSL),  the PSL will have difficulty returning to normal, steady state Pressure after triggering the PSL. In this happenstance, the PSL will have to be bled off manually to return to service.

TAKE HOME MESSAGES: Pressure Switches incorporate "Dry" Pressure Sensors. A change in Pressure sensed by the Pressure Sensor causes the Pressure Switch to change state from Normally Open (NO) to Normally Closed (NC) or vise versa.  Thus, the Pressure Switch can be used for Discrete Control (On-Off Control) of a Process.

The Pressure Switch is either 100% Open (Normally Open = NO) or 100% Closed (Normally Closed = NC). The state of the Pressure Switch will change once the Set Point Pressure is reached. The Pressure Switch will return to its original state once the Reset Point Pressure is sensed.

The outcome of Discrete "On-Off" PV Pressure Control is a cycling above Set Point Pressure and below Reset Point Pressure.  The difference between the Set Point Pressure and the Reset Point Pressure is Deadband. The amount of Deadband desired is specified when the Pressure Switch is ordered. Once the Set Point Pressure is determined and the amount of Deadband is specified, the Reset Point Pressure is likewise determined.

The Fail-Safe Position of a Pressure Switch is the bench state/shelf state of the Pressure Switch. The Fail Safe Position is the position the switch will revert to upon loss of electrical power.

The ISA representation of a Low-Pressure Switch is PSL.  The ISA representation of a High-Pressure Switch is PSH.

The basic parts of a Pressure Switch are:

• The Pressure Sensor
• The Switching Element that contains Normally Open (NO), Normally Closed (NC), and Common (C) Contacts and Terminals

 

Pressure Switches are prevalent in industry. They are used for local alarms or for diverting excess Pressure to Pressure Safety Relief. Differential Pressure Switches will change state when a process pressure increases or decreases. The output from Pressure Switches can be monitored by powerful PLC for rapid decision making.

 

©2022 PTOA Segment 0229

PTOA PV PRESSURE STUDY AREA
PV PRESSURE INSTRUMENTATION

You need to login or register to bookmark/favorite this content.