How to Use a Solenoid Operated Valve to Implement Emergency Shutdown.

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One of the commonest configurations of Solenoid operated valves in use in valve emergency shutdown system is the 3-way valve. These three-way solenoid valves are typically installed in instrument air signal lines, between an instrument air supply to a control valve that is part of an emergency shutdown system.

When used in these emergency shutdown systems, the solenoid valve is energized during normal operation with an electric current and allows the instrument air signal to pass through to the control valve actuator. When the solenoid is de-energised by a remote hand switch, electrical failure or by an automatic emergency trip, the air signal is vented to atmosphere from the control valve and the air supply is isolated. The control valve then moves to its fail safe position on loss of air pressure.

An example of the use of a solenoid valve for emergency signal air switching, in a furnace firing control loop is illustrated below:

During normal operation, the temperature of the heated oil stream is controlled by temperature controller, TC-1, which controls the pressure of fuel gas to the burners indirectly by cascade control of the fuel gas pressure controller PC-2.

If the hot oil temperature exceeds its safe upper limit denoted by, HH (High High) then the high temperature trip instrument TZA-3 de-energizes the solenoid valve in the signal line between PC and its control valve. The control valve then closes to its minimum stop. When temperature falls below the HH limit, the solenoid valve is re-energized permitting fuel gas into the burners for more heating.

How a Solenoid Valve Works

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As shown above, a solenoid valve is made up of two basic functional units. They are
(1) The solenoid, an electro-magnet with a plunger.
(2) A valve containing an orifice in which a disc or plug is positioned to stop or allow flow.
The valve is opened or closed by movement of the magnetic plunger, or core, which is drawn into the solenoid when the coil is energised (an electric current is passed through it). In A above, the solenoid valve opens when energized allowing instrument air to pass through. In B, the valve closes when the solenoid is de-energized preventing instrument air from passing through but vented to the atmosphere
Solenoid valves have a solenoid mounted directly on the valve body with the solenoid core attached to the valve stem. The core is enclosed and free to move in a permanently sealed tube inside the solenoid coil. This construction provides a compact, leak tight assembly.

Solenoid Valve Configurations And Ratings
Solenoids are often used in valve pneumatic controls to activate the valve. In their simplest form, there are 2-way, 3-way and 4-way pneumatic valves. A 2-way pneumatic valve typically has two outlet ports for instrument air flow in and out. A 3-way valve has two outlet ports and one exhaust or vent port. A 4-way valve has two outlet ports and two exhaust ports and it switches air supply between the two outlet ports. 2-way can be normally open (NO) or normally closed (NC), terms that refer to their normal states without power applied.

Solenoid operated valves use an electrical coil to control the position of a poppet, plunger or spool to open or close a valve. Typical solenoid control voltages are 12VDC, 24 VAC/DC, 120VAC or 240VAC.
 

How to Troubleshoot the 4 – 20mA Output of Rosemount 3051 Transmitter

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One of the most versatile and commonly used transmitters is the Rosemount 3051 pressure transmitter. This transmitter can be used to measure pressure, differential pressure, gage pressure, absolute pressure or a liquid level. The Rosemount 3051 transmitter provides an analogue output (4 – 20mA) with a superimposed HART signal on the 4-20 mA output.

The most common problems with the Rosemount 3051 transmitter are adapted from the transmitter manual and can be used to troubleshoot the 4 – 20mA output whenever the symptoms in the table below are observed during transmitter service:

Symptom Corrective Actions
Transmitter miliamp is Zero
  • Verify power is applied to the signal terminals
  • Check power wires  for reverse polarity
  • Verify terminal voltage is 10.5 to 42.4 Vdc
  • Check for open diode across test terminal
Transmitter Not Communicating with Field Communicator
  • Verify the output is between 4 and 20mA or saturation levels
  • Verify terminal voltage is 10.5 to 42.4 Vdc
  • Verify clean DC power to transmitter (Max. AC noise is 0.2 volts peak to peak
  • Check loop resistance, 250 ohms minimum (PS Voltage - transmitter voltage/loop current
  • Have field  communicator poll for all addresses
Transmitter milliamp reading is low or high
  • Verify applied pressure
  • Verify 4 and 20mA range points
  • Verify output is not in alarm condition
  • Verify if 4 - 20mA output trim is required
Transmitter will not respond to changes in applied pressure
  • Check test equipment
  • Check impulse piping or manifold  for blockage
  • Verify the transmitter is not in  multi-drop mode
  • Verify applied pressure is between 4 and 20mA set points
  • Verify output is not in alarm condition
  • Verify transmitter is not in loop test mode
Digital pressure variable reading is low or high
  • Check the accuracy of test equipment
  • Check impulse piping for blockage or low fill in wet leg
  • Verify transmitter is calibrated properly
  • Verify pressure calculations for application
Digital pressure variable reading is erratic
  • Check application for faulty equipment in pressure line
  • Verify transmitter is not reacting to equipment turning on/off
  • Verify damping is set properly for application
Milliamp reading is erratic
  • Verify power source to transmitter has adequate voltage and current
  • Check for external electrical interference
  • Verify transmitter is properly grounded
  • Verify shield for twisted pair is only grounded at one end