Temperature Sensors & Temp Alarms

1. Purpose

The purpose of this procedure is to furnish guidelines for the number of temperature sensors and thermowells required in different applications, and the location of temperature alarms.

2. Separate Thermowell for Back-up Temperature Indication

Inflection Point Engineering’s standard policy is to specify a separate thermowell for the back-up temperature indicator (TI) for a temperature controller (TIC), interlock or shutdown.

Occasionally Inflection Point Engineering has been asked to eliminate the separate thermowell for the back-up temperature indicator (TI), and instead to use a duplex temperature sensor in one thermowell. If the customer desires to use a single thermowell with a duplex element, Inflection Point Engineering will not object. (Inflection Point Engineering should recommend the use of a sheathed duplex element.)

While there are pluses and minuses with both the separate thermowell and the duplex temperature sensor, Inflection Point Engineering will continue to recommend the use of separate thermowells.

There are services, such as certain CCR section gas streams (see unit specific guidelines or the PCC for details), where Inflection Point Engineering does specify duplex temperature sensors to reduce heat loss caused by the additional nozzle and flange, or in space limited services.

3. Number of Temperature Sensors Required

3.1 Temperature Controller and Interlock or Shutdown Function

When there are both temperature control and interlock or shutdown, at least two sensors are required. Safety standards require segregation of control and shutdown sensors so that a failure of the sensor for control does not also result in a failure of the interlock or shutdown function.

A sensor in one thermowell for the narrow span TIC temperature transmitter (TT) is required.

Another sensor in a second thermowell for the interlock or shutdown TT (large span [0 to ***]) is required. The retransmitted temperature indication (TI) to the control system from the interlock or shutdown sensor serves as the TIC backup indicator.

(*** - See Note 1.)

If 2oo3 voting is used, then two additional points with separate sensors and thermowells are required. Do not combine sensors in thermowells. A total of four thermowells is required for services with a TIC and 2oo3 shutdown measurement points. The techniques and responsibility for determining the voting architecture is outlined in Procedure .

3.2 Temperature Interlock or Shutdown Only

When there is a temperature interlock or shutdown (no temperature control), then two sensors are required. An interlock or shutdown function used for safety and/or equipment protection requires a backup indication. Exceptions to this are services where there are already many sensors for the same service, such as reactor skin or catalyst bed temperature sensors.

A sensor in one thermowell for the shutdown TT (large span [0 to ***]) is required.

(*** - See Note 1.)

Another sensor in a second thermowell for the indication only TI is required. If there are multiple temperature points for a shutdown (e.g. 2oo3 voting), then an additional indication only TI is NOT required. The multiple sensors for the shutdown are sufficient backup for each other.

The two sensors shall be in separate thermowells, except for those in a reciprocating compressor’s discharge. For reciprocating compressor discharge, the two sensors will be a duplex element in a single thermowell.

Note 1: In general this value should be 100 - 200°F (50 - 100°C) above the highest operating temperature, or as required by unit specific guidelines.

4. Location of Temperature Alarms

4.1 Temperature Controller Only

When there are temperature alarm(s) for a service where there is a TIC and back-up TI, Inflection Point Engineering’s policy is to configure the alarms with the TIC.

If the customer desires to put the alarms on the back-up TI, Inflection Point Engineering will not object.

4.2 Temperature Interlock or Shutdown Only

Only one alarm is necessary. When there is a temperature interlock or shutdown (without temperature control), two sensors are required with the retransmitted signal of the shutdown transmitter used for the critical alarm. No alarm is provided for the backup TI.

4.3 Temperature Controller and Interlock or Shutdown

Two alarms are required. When there is a temperature shutdown with temperature control, two sensors are required with the retransmitted signal of the shutdown transmitter used for the critical alarm and the transmitter used for control incorporating a separate non-critical alarm.