Loop Mode (1): De-Energized to Safe

Electric based Instrument loop on plant operate in two modes: either energized to safe or de-energized to safe. De-energized to safe mode, also known as fail safe system means that during plant normal condition, there will be electrical current flows through the instrument loop. When trip/shutdown is required, the loop will be de-energized.

See illustration on below picture of a simple unit consisting Pressure Switch High High and SDV. During plant normal condition (pressure is lower than HiHi setting) the switch contact is closed and enables current flow within the loop. Likewise, the solenoid of SDV is energized by electrical power to allow air supply stroking the SDV and then forcing it in open position.

As long as the control system receiving current signal of pressure switch loop, plant is considered as normal. If the pressure increases and reaches HiHi setting, pressure switch will be open and there will be no current flows, hence alarming plant control system.The predetermined executive action shall then be taken to put system in safe condition i.e. de-energize the SDV’s solenoid to make the main valve close.

The reason de-energized to safe mode to be implemented in such system is to make it in safer state (in above case by closing the main valve) when either one of the following conditions occurs:
- Unintended instrument cable disconnection (either pressure switch or solenoid valve)
- Electrical black out

Energized to safe system will be discussed in other post (energized to safe system).

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Temperature Measurement: RTD or Thermocouple?

Two most common temperature instruments used in process industry are RTD and thermocouple. To determine which one to use, the following should be considered:

RTD is preferred if one of the following aspects becomes a concern in measurement: Accuracy, Stability, Sensitivity and Linearity.
Thermocouple is preferred in application for high temperature measurement (more than 400 degC) or when exposed to shock or vibration.

This is the reason why projects usually specify RTD for most process temperature measurement, while thermocouple is applied in heater, flare or for vibration monitoring of compressor or pump.

Cost? It depends on the installation. Some panel or I/O card does not accept RTD directly so it requires a transmitter. On the other hand, extension wire would be an additional cost of thermocouple.

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Instrument Power Cable Sizing

Sizing of instrument cable should be performed to ensure that instrument device such as solenoid valve and its cabling works properly. There are two factors that should be checked i.e. voltage drop and current carrying capacity.


Voltage drop is calculated as follows:

Vdrop = [Rio + (Rw x 2L) + Rins] x In

where:

Rio = Input resistance of I/O Card (Ohm)
Rw = Resistance of wire at specified temperature per lenght (Ohm/m)
Rins = Resistance of instrument device (Ohm)
L = Length of cable / distance from instrument to cabinet in control room (m)
In = Load current (A), obtained from power consumption of instrument device divided by power supply voltage.
Rins, Rio and Rw can be obtained from manufacturer’s catalog for instrument, I/O card and cable respectively.

The cable suits the application if calculated voltage drop is less than maximum permissible voltage drop by the system.

Current Carrying Capacity of a certain cable is defined as the maximum current that can flow through a cable without melting the conductor or insulation. Current Carrying Capacity varies depending on several factors such as conductor size, ambient temperature, and cable installation. These factors are referred as derating factors and IEC has established the values for each.The cable suits the application if nominal current (In) is less than current carrying capacity.
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Solenoid Valve with Manual Reset

When solenoid valve is losing power (de-energized) due to shutdown or other reason, it will actuate to its normal position. On regular solenoid valve, it can be put back to operating condition by energizing the solenoid. While on solenoid valve with manual reset, it will not be so. After being energized, it should be manually resetted prior back into normal operation. So it works like a local permissive.

The necessity of having a manual reset in solenoid valve should be determined during design phase, this shall be identified by process engineer or by operator through HAZOP. The philosophy is based on whether the operator is required to be present on site or not. He/she should check the surrounding condition and make sure everything on site is normal (e.g. no hydrocarbon spill) before resetting the valve. READ MORE >>