Q: I am working with Mott MacDonald O&G Division, Abu Dhabi, as an instrumentation engineer. In my current project, we're using linear heat detection cable. I have noticed that it's available in analog and digital types.
My query is this: In which applications should we consider the use of the analog heat detection cable and in which the digital heat detection cable? Could you please clarify?
A: This type of cable is not used for measurement. It is used for fire protection. The digital version is more like a fuse that melts when a prescribed temperature is met. The analog version is based on either a thermistor or a thermocouple. The thermistor version generally is used once and is done. The thermocouple version, if manufactured with the correct materials, can last through a fire, but is never as good afterwards as a pristine sensor. The thermocouple version is like standard thermocouple sheath material with no hot junction. Instead it has carbon-based filler material that becomes conductive (making a semi-junction) when exposed to extreme heat. Neither of these will tell you where along the length the fire is located and, in any case, is unsuitable for temperature measurement.
Frank L. Johnson
A: In our projects, we used digital-type, heat-detection cable for some storage tanks and cellars of electrical substations. Digital cable can work for high temperatures and long distances (10,000 m) as compared to analog cables (1000 m). There is a site where you can find more information.
Harvindar S Gambhir
Q: As shown in Figure 1, I have to do an independent partial valve stroke test per IEC 61511. I have to do this test for both a PLC/DCS controller and SOV combinations, and a partial stroke test for pneumatic actuator and valve combinations.
What are the available methods to perform these partial stroke tests for my ESDV valves?
A: There are two tests that need to be performed:
- Partial stroke of the valve. This is done through a command from the PLC, which moves the valves slightly and gets feedback that valve has moved from 100% to 90% in general.
- Health of the SOV. This is to check that the SOV is opening by a pulse command from a PLC, and getting feedback of reduction of pressure on the actuator.
Harvindar S Gambhir
A: There are various solutions for partial stroke testing offered by different manufacturers. Some of them are mechanical and some are electronic. Metso, Emerson, Rotork, etc., provide electronic solutions. Please note that not all can provide diagnostics for SOV. You can refer to the VG9000 catalog for further detail.
A: There is no requirement in IEC 61511 for partial stroke testing of valves. However, if you wish to do it, there are over a dozen vendors with solutions that differ in many ways. The standards do not mandate that level of detail.
A: I would suggest an online search on partial stroke valves. Several vendors have addressed the issues and offer hardware to make the partial stroke tests. Simply switching the valve and hoping to manually stop the valve is very likely to seriously upset the production people. This subject has been discussed and addressed for a decade. It is not new.
A: What is the correct way of representing an electronic remote sensor of a differential pressure measurement transmitter on a P&ID diagram? This question involves a Rosemount electronic remote sensor level transmitter. Rosemount supplied two transmitters with diaphragm seals that we mount to the vessel. The two transmitters communicate via a proprietary bus signal, and generate a single 4–20 ma signal which is representative of the vessel level and is fed to the control system.