Dials, control-valve

This formidable term has a surprisingly simple definition. Suppose the proportional band is set on 100%. Now the set-point dial on the front of the controller is moved full scale. The corresponding control valve outside will then also move full scale. 

Certain process parameters respond slowly to changes in control-valve position. A furnace outlet temperature controller is an example. Even when the fuel-gas control valve swings wide open, the furnace tubes themselves take awhile to heat. Such controllers (often on temperature) contain a rate dial. As a process variable moves away from the set point, rate increases the controller gain (reduced proportional band) as a function of the speed with which the process is moving away from the set point. This will help... 

Controllers may be switched from the auto to the manual mode. When a controller is in manual, the set-point dial can be moved without affecting the control valve. Operating personnel must then adjust a different dial on the controller to provide a steady signal to the control valve. This is called open loop control, since to have control the operator must move the manual dial to bring (he measurement to the desired value. 

What does this mean in the field If the controller is responding too slowly (that is, the process is changing too quickly for the control valve to keep up), the proportional band should be dialed down. This will speed up the reaction time of the control valve. 

If the intrinsic gain of a system exceeds 100%, control instability can result. In the field, this means that one cannot arbitrarily keep speeding up a control valve s response time by dialing down the proportional band. A flow control loop with a gain of over 100% would likely have a flow recorder chart as in Figure 28—4. 

Figure 38.4 shows a control valve installation in the field. Next to the control valve there will be a small cabinet with three dials displayed ... 

The Signal and Output dials should have the same reading. Let s assume that this is an "air-to-close valve." That means that more air pressure closes the valve. Typically, the supply air pressure is 30 psig. If the air output pressure to the control valve is 3 psig, the valve should be 100 percent open. If the air output pressure to the control valve is 15 psig, the valve would be shut. We say the valve operates... 

For many (but not all) control valves, you can tell if air pressure is supposed to open or close the valve. If the tubing to the diaphragm is on the top, as shown in Fig. 38.4, then the air pressure is usually (but not always) going to close the valve. When in doubt, ask the panel operator to move the valve. Touch the stem with your gloved finger and see what happens to the air pressure of the output dial. 

Facility is provided to bring the equipment up to the desired absolute pressure without subjecting the test unit to excessive pressure difference between its interior and exterior. This is accomplished by first filling the test cell with water by means of a hand operated hydraulic pump(17 in Figure 1) to a suitable value as indicated on the Bourdon dial gauges(P in Figure 1). The pressure thus developed is used also to control the appropriate back-pressure relief valve... 

Control dials, pump and timer parts, valves, gears... 

The gas flow rate from the cylinder is controlled by the cylinder head regulating valve (Fig. 18.1). Before you start make sure that the regulator outlet tap is off (turn anti-clockwise until it feels free ) and then open the valve to the cylinder with the cylinder spanner (turn anti-clockwise) and the cylinder pressure should be indicated on the right-hand pressure dial. Switch on the gas at the regulator (turn slowly clockwise) until there is a reading on the left-hand dial. Use the minimum pressure required to provide a steady flow of gas. The gas flow rate from the regulator can be controlled further by a needle valve on the regulator outlet, if one is fitted. To switch off, reverse the instructions above. 

A hydraulic dash-pot Superficial Hardness Test Machine with a l/l6 in. spherical indenter and a 15 kg major load is utilized in this test. The dash-pot is an oil reservoir that controls the rate of application of the major load. The dash-pot time is the time required to empty the dash-pot and apply the major load to the sample. This dash-pot time may be adjusted by means of a needle valve. Because the plastic materials are susceptible to deformation, they are indented further than metals. A small difference in dash-pot time can result in wide variations of hardness readings. For this reason the dash-pot time must be determined to the nearest 0.1 sec and reported with the test results. Dash-pot times of 3.5 to 4.0 sec are satisfactory. A convenient way to measure dash-pot time is by measuring the time the major load application handle is in motion. After releasing the major load, a period of 10 sec is allowed to elapse before the reading is taken. The dial indicator may not be stationary upon the completion of this 10 sec interval, indicating elastic recovery of the sample. 

In principle, the controller functions like a human process operator who watches the dial of an instrument indicating the actual value of a process variable, e.g., a rate of flow. When the pointer moves away from the desired value, the operator corrects this by adjusting a valve (the correcting element) so as to restore the flow rate to its desired value. In modern industry the control functions are automated, using electronic equipment. Pneumatic control systems, which were used to some extent in the cement industry in the past, are now virtually obsolete. 

The purpose of step 1 is to understand where people fit into the system goals and functions. What are some of the assumptions people have toward the system Is the system highly computer controlled Do operators need to read dials and operate valves Focus on the hazards identified in the other system safety analyses. Identify which of those are human induced or can be mitigated by human actions. 

Operation—The plant operations personnel need room to walk safely around the machine. They must have access to valves, switches, and gauges and must be able to see all gauges, lights, and dials on the control panels. 

Pressure measurement for piping and vessels is achieved by the installation of dial indicators or by distributed control systems. The piping and vessels are tapped at the required location and furnished with a 3/4-in threaded or flanged conneaion and a block valve. Tbe dial indicator is screwed into the block ralve if a remote readout system is required, the valve becomes the sensing connection. Like the thermocouple, the dial can be either fixed or swivel-headed to fecilitate readout. A dual local indicator and transmitter s) em needs only one tapping point. Exhibit 14-9 depicts a typical pressure gauge and a dual-pressure system hookup. 

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