Control valves poor design

Relief valves are often seen to be undersized for the required relieving rate, due either to poor initial design or changes in the process conditions which occurred during design. The most common system problem is that the relief valve was adequately sized for blocked discharge but not sized for the flowrate that could occur as a result of a failure in the open position of an upstream control valve (i.e., gas blowby). See Chapter 13. 

Many accidents, particularly on batch plants, have been due to runaway reactions, that is, reactions that get out of control. The reaction becomes so rapid that the cooling system cannot prevent a rapid rise in temperature, and/or the relief valve or rupture disc cannot prevent a rapid rise in pressure, and the reactor ruptures. Examples are described in the chapter on human error (Sections 3.2.1 e and 3.2.8), although the incidents were really due to poor design, which left traps into which someone ultimately fell. 

Inadequate ergonomic design in areas such as control panels and the labeling and placement of valves on the plant can also be regarded as a latent failure because it will increase the probability of active errors. For example, a worker may misread process information from a poorly designed display. Poorly labeled and situated valves can cause the wrong valve to be selected, with possibly disastrous consequences. 

The valves supplied with these pumps showed an erratic behavior at low flow rates, mainly due to a too large diameter of the valve balls and poor design of the valve seats and valve chambers. New pumpheads and valves were therefore developed, manufactured and mounted on the pumps. The heads and fittings were made from PVDF with valve seats and 3 mm diameter balls in acid resistant steel. On these pumps the stroke length has to be set mechanically and this set-screw was modified so that it could be turned by a simple manipulator, if necessary. The electronic frequency control was modified for remote operation. 

Obviously the example was designed to display the range of calculations needed with a control valve carrying a liquid, and is to that extent artificial. But it should not be thought that similar conditions are never found in practice it is quite possible for a large process plant to contain such an instance of poor installation, particularly if it has undergone a number of modifications over the years, at the hands of different teams with different objectives. 

However, if the design control valve pressure drop is small, the increase in flow resulting from opening the valve will be small. This translates as poor controllability because only... 

If we cannot open the discharge control valve of a centrifugal pump 100 percent before the FLA point is reached, then we say that the pump is driver-limited. This is a frustrating problem for plant operators, and clearly reflects poor design practice in undersizing the motor driver horsepower. 

Reverse osmosis skids are typically contained within a frame of galvanized or urethane-coated steel. Skids should be designed for easy access for monitoring and maintenance. Access to controls, instruments, valves, the pump and motor, and membranes is essential. Access to the permeate from each pressure vessel is often overlooked. Without such access, profiling and probing used to troubleshoot poor performance is not possible (see Chapter 14.7). 

Usually elaborate control systems cannot correct for problems such as those caused by a (1) worn screw and barrel, (2) inadequate drive torque, or (3) poor screw design. For example, such systems will not yield good temperature control unless all features essential to good control are well maintained. Burnt-out heating elements cannot be tolerated. Another common deficiency for liquid-cooled extruders is fouling or restrictions in the plumbing system or inoperative valves. Other factors of these types also exist. 

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