Open Discussion Forum Question 5

5. Can you tell me your preferred way to measure differential pressure across a distillation column? Examples: Pressure transmitters at each point with a calculation in the D.C.S. A differential pressure transmitter located on top of the column - US Gulf Coast Refiner - 21 August 2003

Comment from Washington Distillation Consultant - 26 August 2003

For most refinery services I recommend a differential pressure located at the top of the column with impulse legs with sufficiently large diameter to allow for free draining of liquid back to the column. For most services steam tracing is recommended on the impulse legs to prevent condensation. The practice of using glycol filled legs with the pressure instrument located at the lower connection tap is to be avoided. The purpose of this arrangement is convenience for maintenance and ease of installation. However, the glycol leg filled system has had many operating problems in a majority of plants. The main objective of the DP system is accurate and usable measurements. The system needs to meet that objective. Mounting the pressure gauge with free draining legs is the best method. For high vapor density systems with low pressure drops don't forget to take the vapor density into account.

For towers with a large number of trays (giving a high relative pressure drop) at modest or low pressures, using two accurate pressure measurement devices with a differential pressure calculated in the control computer may prove acceptable.