Pressure-control valve, opening

Another example of pressure control by variable heat transfer coefficient is a vacuum condenser. The vacuum system pulls the inerts out through a vent. The control valve between the condenser and vacuum system varies the amount of inerts leaving the condenser. If the pressure gets too high, the control valve opens to pull out more inerts and produce a smaller tube area blanketed by inerts. Since relatively stagnant inerts have poorer heat transfer than condensing vapors, additional inerts... 

Industry literature typically cites concern with open air explosions when 4,536 kgs (10,000 lbs.) or more of flammable gas is released, however, open air explosions at lower amounts of materials are not unheard of. When the release quantity is less than 4,536 kgs (10,000 lbs.), a flash fire is usually the result. The resulting fire or explosion damage can cripple a hydrocarbon processing facility. Extreme care must be taken to prevent the release of hydrocarbon from vessels resulting in vapor releases and resultant blast overpressure. Measures such as hydrotesting, weld inspections, pressure control valves, adequate pressure safety valves, etc., should all be prudently applied. 

Monthly Gauges in good working condition and reading normal pressure, control valves (with locks or electronic supervision) open, fire department connections in good condition... 

A controller opens and closes the pressure control valve at the gas outlet to maintain desired vessel pressure. Normally, horizontal separators are operated half full of liquid to maximize the gas-liquid interface area. 

I well remember one pentane-hexane splitter in Toronto. The tower simply could not make a decent split, regardless of the feed or reflux rate selected. The tower-top pressure was swinging between 12 and 20 psig. The flooded condenser pressure control valve, shown in Fig. 3.1, was operating between 5 and 15 percent open, and hence it was responding in a nonlinear fashion (most control valves work properly only at 20 to 75 percent open). The problem may be explained as follows. 

To consider a third case, we wish to maintain the original 240°F shell-side temperature, but to increase the steam flow from 10,000 to 15,000 lb/h. This will force the steam inlet control valve to open. As the control valve opens, the pressure in the channel head rises from 100 psig to the full steam header pressure of 160 psig. At this pressure, steam condenses at 360°F. The new AT is then (360°F - 240°F) = 120°F. This new temperature driving force is 50 percent greater than the case one driving force of 80 percent. Hence the rate of steam condensation also increases by 50 percent, from 10,000 to 15,000 lb/h. 

On the surface, this story sounds crazy. But, let s see what happened. This deaerator had been designed for a much smaller flow of 160°F BFW, and a much larger flow of hot-steam condensate, than are current operations. The cold BFW feed line had been oversized, but the steam line was of marginal size. As the demand for hot BFW increased, the cold-BFW level-control valve opened. This reduced the temperature and pressure in the deaerator drum. In response, the steam pressure-control valve also opened. But when the cold-BFW level-control valve was 40 percent open, the steam pressure-control valve was 100 percent. Steam flow was now maxed out. 

As the hot-vapor bypass valve opens, the condensate level in the shell side of the condenser increases to produce cooler, subcooled liquid. This reduces the surface area of the condenser exposed to the saturated vapor. To condense this vapor, with a smaller heat-transfer area, the pressure of condensation must increase. This, in turn, raises the tower pressure. This then is how opening the hot bypass pressure-control valve increases the tower pressure. 

When the flow of softened water exceeded 80,000 lb/h, the heating steam pressure-control valve, shown in Fig. 15.1, would open to 100 percent. 

But, dear reader, do not forget that when the steam inlet pressure-control valve is 100 percent open, any further increase of cool softened-water flow will suppress the deaerator s pressure. When the pressure of a vapor goes down, its volume goes up ... 

As regards Type C apparatus, typical of CCNY s setup (Yerushalmi et al, 1976), when the solid rate controlling valve opens but slightly, and the initial solids inventory is high, the voidage profile will be close to that for Type A apparatus, while when this valve is mostly open, the voidage profile will be similar to that for Type B apparatus. In most casts, its axial voidage variation will be affected by the initial solids inventory. The effect of solids inventory is reflected in what Weinstein et al. (1984) called imposed pressure drop across the fast bed. ... 

The change in liquid control valve opening will cause an increase in liquid flow due to the increased flow conductance of the valve, but the increased valve opening will also lead to a decrease in differential pressure, which will tend to reduce the flow. The two contributions have been labelled IVand to indicate their origin, and the total flow change is the sum of the two ... 

Safety features like a steam pressure control valve, air vent, side bottom drain, and arrangement to open the unit (for cleaning) either manually or through hydraulic system were provided in later versions. 

This is obviously a time-consuming method, and automatic collecting and measuring equipment is now available. In the automated gas burette developed by ICI (Figure 3.10), the liquid height is measured by a pressure transducer. After a preset volume of gas has been collected, a solenoid-controlled valve opens to empty the tube. The output from the transducer is recorded continuously and provides a measure of the gas evolved. Lambert and Amery describe a thermal mass flowmeter which records gas flow automatically, is simple to use and can handle most gases including those which are corrosive. 

The RO unit contains only one user controllable function, the control of the final reject flow rate. The reject flow control valve (50% open) is used in conjunction with the RO feed pressure control valve (25—30% open). The reject flow valve and the RO feed pressure valve are throttled to achieve the proper operating pressure necessary to achieve the desired product water recovery. These control valves may require fine tuning to achieve the desired productivity. In addition, as the membrane performance declines with time due to changes in the material properties of the polymer, the control valve settings win require adjustments, when higher feed pressures are required. 

The viscosity of the hydraulic oil can severely affect injection pressure. While hydraulic oils are fairly Newtonian, their viscosities will decrease with increasing temperature. Decreases in oil viscosity generally affect pressure settings pressure develops more rapidly in the hydraulic lines, causing pressure control valves to open prematurely. 

If the control valves open during the high pressure (25 Barg) phase, the pressure would instantaneously rise in the vent line, so the SIF would need to operate within seconds to prevent a pressure transient in the downstream equipment. The design could be feasibly implemented with 2 pressure transmitters and 2 isolation valves. The required response time could be met using quick vents (fast-acting solenoids) to rapidly close the isolation valves. 

Process technicians use hydraulic systems (see Figure 9-6) to open or close valves, lift heavy objects, run hydraulic motors, and stop the rotation of a rotary or reciprocating device. A hydraulic system is a collection of equipment designed to apply pressure on a confined liquid in order to perform work. A similar process is used in the brake systems of most cars and trucks. A hydraulic system is composed of a fluid reservoir, strainer, pump, piping, flow control valve, pressure control valve, four-way directional control valve, and actuator (cylinder, piston). 

Minor—any event that may cause minor injury or minor system damage, but does not significantly impact the mission (e.g., pressure control valve fails open, causing pressure drops and increased caustic levels)... 

Pressure-control valves have a third port to vent excess pressure and prevent it from affecting the downstream piping. The bypass, or exhaust, port has an internal flow-control device, such as a diaphragm or piston, that opens at predetermined set-points to permit the excess pressure to bypass the valve s primary discharge. In pneumatic circuits, the bypass port vents to the atmosphere. In hydraulic circuits, it must be connected to a piping system that returns to the hydraulic reservoir. 

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