Instrumentation looping

Instrument Loop-A combination of one or more interconnected instruments arranged to measure or control a process variable. 

Ensure that detailed control parameters have been established and recorded for each instrument loop... 

Once a critical instrument loop is identified, a procedure for testing the entire loop must be written. The test procedure will influence the design of the new system, since, if possible, the test should be an actual performance test. For example, if a high temperature should close a valve, the ideal test would consist of raising the temperature to see if the value closes. Efforts should be made to avoid test procedures which require temporary wiring disconnects, valve closures, and so on, which might not be returned properly to operating condition. 

A minimum interval of three months between tests of each critical instrument loop is recommended in no case should the interval exceed 12 months. However, frequency of testing should be based on plant judgment and experience, as well as on Technology Center information. 

Figure 10 was prepared by a Dow technology center as a logic diagram to illustrate an approach to be taken to determine how often a critical instrument loop should be tested. 

As described above, performance testing is an important consideration in the design of a critical instrument loop. Components of the system must be selected for ease of testing, as well as for their ruggedness and reliability. 

In computer systems, program logic is an important component of the critical instrument loop. Security precautions must be taken to prevent inadvertent changes in programming. In addition, manual overrides are discouraged. For example, if it is critical to shut down a pump during an emergency situation, a manual on switch may not be desirable. 

Drawing register Loop schedule Instrument data sheets Instrument loop schematics Logic and interlock diagrams Wiring diagrams... 

Calibration of the instrumentation will be performed over the complete instrument loop. During each loop calibration, all data must be documented on appropriate instrument and loop calibration sheets and submitted to the pharmaceutical manufacturer for review, approval, and record. Calibration test equipment must be traceable back to agreed-upon national standards and documented on each calibration result sheet. 

A CSPRS is, in fact, a spring-operated SRV which can operate as stand alone but in normal operations is controlled by an actuator (usually pneumatic or hydraulic) and which opens upon the signal of an instrumentation loop. Due to the additional force on the spring, more accurate set pressure can be... 

If a 100,000 heat exchanger is to be added to an existing crude unit, the cost of installing it will normally be far less than the 230,000 calculated from Table 8.2. This is because the sewers, control room, and instrument loops are already in place. The foundation, piping, and insulation of the exchanger might cost 80,000 to 160,000. It is simply not possible to estimate an installed cost of one or two items of process equipment without considering each case on an individual basis. 

In earlier times, costs associated with business interruption (which can exceed property losses) after an interlock failure were largely ignored and those interlocks were often placed with the so-called operational interlocks. McMillan s approach was a system that grouped the protection instrument loops into four classes. These are the classes and a rough idea of the number of affected loops ... 

At PPG Industries in Lake Charles, Louisiana, numerous instrument loops provide critical safety, alarm, and shutdown functions. These protective instruments are located on reactors, oil heaters, incinerators, cracking furnaces, compressors, steam-heated vaporizers, kettles, distillation columns, boilers, turbines, and other critical equipment. Process analyzers and flammable vapor detectors also enhance the overall process safety environment. [8]... 

Instrument loops serving equipment described above can function in either an on-line or a standby manner both types can fail. Failure of an on-line loop, such as a failure of level control valve, becomes known rather quickly when the operation deviates either gradually or drastically from the normal. Depending on the type of failure, this may place a demand on the standby loop. 

After all, failure of a standby instrument loop, such as a alarm or safety interlock, will not become evident until a potential hazard is detected. Potential defects developing in these loops must be discovered by periodic prooftesting. 

It takes years to develop and fine-tune a prooftest program. Time is required to identify all of the instrument loops that need to be included, and more time is needed to systematically collect or develop data sheets containing basic operational information for each device or loop. During the development stage, test methods need to be defined, and the appropriate personnel responsible for testing must be identified and trained. Finally, test frequencies must be decided. 

Some process safety instrument loops are more critical than others. The importance class and test frequency are initially assigned by the group(s) directly associated with the origin and the reason for the required test. Input is received from a variety of in-plant departments such... 

These are instrument loops that are necessary to avoid perilous situations. These critical consequence instrumentloops are those whose failure would either cause, or fail to inform of, situations resulting in accidental fire, explosion, uncontrolled release of dangerous materials, personal injury, or death. The OSHA Process Safety management loops fall within this group. 

These serious consequence instrument loops are those instrument systems whose failure could cause, or fail to inform of, serious conditions involving environmental releases, major property or production losses, or other non-life-threatening situations. These alarms are given a slightly lower priority but are also prooftested on a regular schedule. These prooftests may be required by many of the originators listed for Class 1 Prooftests. 

Figure 10-23 A chemical process operator relies on a furnace fuel shutdown system. Class 3 Normal Consequences Instrument Loops...

Assigning prooftest frequencies for complex, safety-instrumentation loops requires sound engineering judgment for simple systems. For more complex, interlock systems, the frequency is a function of the tolerable hazard rates. For example, DuPont Sabine River Works (Orange, Texas) reported it had 35,000 instruments in service. Every safety interlock is... 

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

The calibration procedures adopted on-site must be agreed on with the customer and conform to recognized industry instrument calibration standards and the supplier s instructions. These procedures must be applied to all in-line instrumentation, loop instrumentation, local controllers, analyzers, and so on. Where the control and monitoring instrumentation is integrated with a computerized control system and where factory tests have been carried out, the installation calibration procedure should be agreed on with the customer. 

The loop test results should be recorded (a sample instrument loop check sheet has been provided for reference purposes in Appendix 23D) and included with the site test records. Checks for mechanical/electrical completeness are recorded using the upper section of the sheet and the dytramic loop test results are recorded on the lower section. The test results sheets will provide the documentary evidence essential for Installation Qualification (IQ). Representatives from the installation contractor and/or the customer will witness the final loop tests and countersign the test sheets. Any tests not witnessed must be accompanied by written confirmation from the customer that witnessing has been waived. 

Before this level of testing commences, the instrument loop must be prepared in accordance with the loop testing procedure. All test methods must be agreed with the customer. 

Reviews of algorithms and calculations (smart instruments, loop controllers)... 

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