Instrumentation - Pressure
August 21, 2001

Introduction

Pressure measurements are taken where ever th information will be useful for controlling, monitoring, or trouble shooting the process or equipment.   Critical pressures should be available in the control room and if appropriate alarm levels set to inform operators when the pressure is outside its normal range.

Pump discharge pressures allow the operator to determine pump performance by taking pressure measurements and estimating flows from pump curves.   Differential pressures if monitored and recorded over long periods of time can indicate when a catalyst bed, heat exchanger, column, filter, etc. is fouling or plugging up.

Installation

Pressure gauges should always be mounted with an isolation valve so the gauge can be removed safely during operation.  The connection for the gauge is 1/2" NPT.  The connection to the process line and valve can be any size as long as the connection is reduced to 1/2" NPT for mounting the gauge.

Ranges

Standard ranges are given below.  When specifying ranges the closes standard range should be used that adequately covers the anticipated operating range (ie. minimum to maximum).

Chemical Seals

Chemical seals should be considered where:

• the process medium will corrode the transmitter connection and pressure sensing element.

• the process medium contains suspended solids or is sufficiently viscous to clog the impulse piping.

• the process temperature is outside of the normal operating limits of the transmitter and cannot be brought into those limits with impulse piping.

• the process medium might freeze or solidify in the transmitter or impulse piping.

• sanitary or aseptic conditions must be maintained.

• there is a need to eliminate the maintenance requirements associated with wet leg applications.

Based on the above guidelines, chemical seals are required on all strong acid, weak acid, molten sulphur and wet process gas services.  Utilities such as air, water, steam, cooling water, process water will not require chemical seals.

Chemical seals come in a variety of materials including metals, PTFE, and elastomers.  Metals such as 304 SS, 316 SS, Hastelloy C (alloy C-276), tantalum, Carpenter 20, Nickel 200, Monel 400, Inconel 600 and titanium are available.  Elastomer materials include viton, Buna N and neoprene.