Temperature transmitter

Both of the sources above contain tWo types of failure rate data used in CPQRAs time-related failure rates and demand-related failure rates. Time-related failure rates, presented as failures per 10 hours, are for equipment that is normally functioning, for example, a running pump, or a temperature transmitter. Data are collected to reflect the number of equipment failures per operating hour or per calendar hour. 

For the sake of illustration, let s presume that the temperature transmitter has a built-in amplifier which allows us to have a measurement gain of Km = 5 mV/°C. Let s also presume that there is no transport lag, and the thermocouple response is rapid. The measurement transfer function in this case is simply... 

The temperature control loop consists of a temperature transmitter, a temperature controller, and a temperature control valve. The diagonally crossed lines indicate that the control signals are air (pneumatic). 

The lube oil temperature is the controlled variable because it is maintained at a desired value (the setpoint). Cooling water flow rate is the manipulated variable because it is adjusted by the temperature control valve to maintain the lube oil temperature. The temperature transmitter senses the temperature of the lube oil as it leaves the cooler and sends an air signal that is proportional to the temperature controller. Next, the temperature controller compares the actual temperature of the lube oil to the setpoint (the desired value). If a difference exists between the actual and desired temperatures, the controller will vary the control air signal to the temperature control valve. This causes it to move in the direction and by the amount needed to correct the difference. For example, if the actual temperature is greater than the setpoint value, the controller will vary the control air signal and cause the valve to move in the open direction. 

B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lube oil cooler is the plant in this example, and its controlled output is the lube oil temperature. The temperature transmitter is the feedback element. It senses the controlled output and lube oil temperature and produces the feedback signal. 

The temperature transmitter has a range of 50 to 250 F, so its output pneumatic pressure signal goes from 3 psig at 50° F to 15 psig at 250°F. 

Figure TAb shows a temperature transmitter which accepts thermocouple input signals and is set up so that its current output goes from 4 to 20 m A as the process temperature varies from 0 to 250 F. The range of the temperature transmitter is SO to 2S0"F, its span is 200 F, and its zero is 50 F. The gain of the temperature transmitter is...

Next we look at the temperature transmitter. It is direct acting (when the process temperature goes up, the transmitter output signal, PM, goes up). Now if PM increases, we want to have less steam. This means that the controller output must decrease since the valve is AO. Thus the controller must be reverse-acting and have a positive gain. 

The temperature transmitter on the process oil stream leaving the heat exchanger has a range of 5 -150°F. The range of the orifice-differential pressure flow transmitter on the chilled water is O-ISOO gpm. Alt instrumentation is electronic (4 to 20 mAV Assume the chilled-water pump is centrifugal with a flat pump curve. 

Reactor temperature transmitter range SO-250 F Circulating jacket water temperature transmitter range 50-1 SO F Makeup cooling water flow transmitter range 0-250 gpm (orifice plate + diflerential pressure transmitter) >... 

Figure 8.4fc shows another type of selective control system. The signals from the three temperature transmitters located at various positions along a tubular reactor are fed into a high selector. The highest temperature is sent to the temperature controller whose output manipulates cooling water. Thus, this system controls the peak temperature in the reactor, wherever it is located. 

The span of the temperature transmitter is 100-200°F. Control valves have linear trim and constant pressure drop, and are half open under normal conditions. Normal condenser flow is 30 gpm. Normal jacket flow is 20 gpm. A temperature measurement lag of 12 seconds is introduced into the system by the thermowell. 

We must include the 0.S minute lag of the temperature transmitter and the gains for both the transmitter and the valve. 

The control valve on the steam has linear installed characteristics and passes 500 Ibi min when wide open. An electronic temperature transmitter (range 50-250°F) is used, A lemp>efature measurement lag of 10 seconds and a heat trans fer lag of 20 seconds can be assumed. A proportional-only temperature contioller is used. 

Filled-bulb temperature transmitter. (Courtesy of Moore Products.)... 

Other supervisory signals may come from fire protection system components such as supervised control valves, system air and supervisory air pressure transmitters, water tank level and temperature transmitters, valve house and fire water pump building temperature transmitters, and fire water pumps. 

FIG. 4. (A) Single-plotted event records from 2 rats. Cage activity counts per 10 min were derived from abdominal body temperature transmitter... 

A relay-feedback test on the reactor temperature controller is used to obtain the ultimate gain and frequency (K, = 64 and Pv = 10 min), using a 50 K temperature transmitter span and assuming the maximum cooling water flow is twice the steady-state value. The Tyreus-Luyben settings give oscillatory response, so the controller gain is reduced by factor of 2 (Kc = 10, t = 1320 s). 

There are two controllers. The proportional reactor level control has a gain of 5. The reactor temperature controller is tuned by running a relay-feedback test. The manipulated variable is the cooling water flowrate in the condenser. With a 50-K temperature transmitter span and the cooling water control valve half open at design conditions, the resulting tuning constants are Kc = 4.23 and = 25 min. 

The effect of heat transfer area is illustrated in Figure 4.3. Three different areas are used. The temperature controller is proportional with a gain of 0.1 (dimensionless using a 50-K temperature transmitter span and split-range hows shown in Figure 2.1). The set-point is ramped to 340 K in 60 min. Clearly in the numerical example, a jacket-cooled batch reactor of the size selected (2 m diameter) and with the given heat of reaction would produce runaway reactions. An external heat exchanger with 4 times the jacket area would be required to catch the reaction. 

Two temperature controllers are used. The first manipulates the flowrate of the A feed. A 45 min ramp in this reactor temperature controller is used with Kci = 0.5 and Tj2 = 20 min. Two 30-s lags are included in the loop. The span of the temperature transmitter is 50 K, and the maximum flowrate FA0 of the reactant A is 0.004 m3/s. The second temperature controller setting the flowrates of the hot and cold streams to the jacket is proportional-only with a 330 K setpoint and a gain of 0.05. The maximum cold water and hot water flowrates are 0.005 and 0.002 m3/s, respectively. 

Two 1-min temperature measurement lags are included in the temperature control loop. A 50°C temperature transmitter span is used. Controller gain is 10, and integral time is 10 min. 

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