Impulse lines, instrumentation

A small block valve is usually installed at a tank on any instrument impulse line just in case a leak develops on the small tubing. In this unit block valves are generally avoided on overpressure or vacuum relief devices. If a block valve that can defeat an overpressure device cannot be avoided, such a valve is chained and locked open or car-sealed opened and a protocol is developed to administratively control accidental closing of such a prime safety feature. 

If an instrument or instrument system malfunctions, the operator tries to correct the problem. If an alarm, flow- or temperature-measuring instrument or shutdown system malfunctions, the operator s first response should be an attempt to restore it to service. It may be a plugged impulse line, inadvertent flow shut off to an analyzer, etc. If the Instrument System is a Class 1 or 2, immediately go to Step 2. If the alarm is a Class J. the Lead Operator (or Shift Foreman) should be consulted and follow established procedures and his knowledge of the unit and the specifics of the situation to determine the temporary steps required. At the minimum, a note in the maintenance logbook must be made to initiate repairs. Stop Here for Class 3 Alarms. 

The prime consideration for instrument connections is to avoid hydraulic interference in the column or impulse line, which would lead to erroneous measurements or instrument malfunction. False information supplied by instruments has been the cause of premature flooding, column damage, and poor separation in many columns. This chapter examines the preferred practices, reviews common pitfalls, and supplies guidelines for avoiding pitfalls with column instrument connections. 

The connection(s) must be compatible with the instrument and permit correct installation of impulse lines. Figure 5.36 and c shows a case where overlooking this guideline prompts incorrect readings (see guideline 3 in Sec. 5.3 for details). Defective impulse line piping is one of the most frequent sources of instrument problems. 

All instruments, except for those that are directly required for monitoring the line-blowing operation, should be disconnected or at least blocked off dming line blowing to avoid debris entering the impulse lines. With the impulse-lines disconnected, the tie-ins of the instruments to the lines should be blown free of debris just before blowing a line is terminated. 

Troublesome impulse lines This is one of the most common instrument problems. Impulse lines are discussed at length in Chap. 5. 

Mr A. Forbes Gower raised a question on the BCD samples. It would not be feasible to use the Venturi tappings to withdraw a BCD sample as the flow through the Impulse line would introduce a pressure Imbalance and invalidate the instrument reading. It is essential to vent the Impulse lines to remove the gases and even occasional use for sampling would lead to... 

A process impulse line is the connection between a process fluid contained in piping or in a vessel and a measuring or control device. Therefore the materials used must be compatible with the resulting conditions of pressure and temperature and the nature of the fluid. Special attention is required concerning the configuration of the piping. When the process line is being flushed and pressure tested, precautions must be taken to isolate the instrument. The impulse line should be flushed and pressure tested separately. 

Ideally the field instrumentation and process connections — for example, impulse lines for dp cells — should have been properly installed prior to commissioning as should the infrastructure such as power and air supplies, con-duits/trunking/cable trays, wiring of termination cabinets and tags. In practice this is rarely so, and the primary objective of the pre-conunissioning process is to identify faulty installation. If quality assured procedures have been used by the installation contractor, credit may be taken for this and some of the precommissioning checks omitted. 

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