Control valve flashing

The two portions of the feed stream recombine and flow into the high pressure separator where the Hquid is separated from the vapor and is fed into an intermediate section of the demethanizer with Hquid level control. The decrease in pressure across the level-control valve causes some of the Hquid to flash which results in a decrease in the stream temperature. The pressure of the vapor stream is decreased by the way of a turboexpander to recover... 

FIG. 8-78 Generic depictions of average pressure at subsequent cross sections throughout a control valve, FiS selected for illustration are 0,9 and 0,63 for low and high recovery, respectively. Internal pressure in the high-recovery valve is shown as a dashed line for flashing conditions [Pg.788]

When a pulsation frequency coincides with a mechanical or acoustic resonance, severe vibration can result. A common cause for pulsation is the presence of flow control valves or pressure regulators. These often operate with high pressure drops (i.e., high flow velocities), which can result in the generation of severe pulsation. Flashing and cavitation can also contribute. 

Across a control valve the fluid is accelerated to some maximum velocity. At this point the pressure reduces to its lowest value. If this pressure is lower than the liquid s vapor pressure, flashing will produce bubbles or cavities of vapor. The pressure will rise or recover downstream of the lowest pressure point. If the pressure rises to above the vapor pressure, the bubbles or cavities collapse. This causes noise, vibration, and physical damage. 

When there is a choice, design for no flashing. When there is no choice, locate the valve to flash into a vessel if possible. If flashing or cavitation cannot be avoided, select hardw are that can withstand these severe conditions. The dowmstream line will have to be sized for tw o phase flow. It is suggested to use a long conical adaptor from the control valve to the downstream line. 

For sizing a flashing control valve add the Cv s of the liquid and the vapor. 

The most frequently encountered flashing problems are in control valves. Downstream from the control valve a point of lowest pressure is reached, followed by pressure recovery. A liquid will flash if the low pressure point is below its vapor pressure. Subsequent pressure recovery can collapse the vapor bubbles or cavities, causing noise, vibration, and physical damage. 

A vapor poeket on the exchanger s low-pressure side can create a cushion that may greatly diminish the pressure transient s intensity. A transient analysis may not be required if sufficient low-pressure side vapor exists (although tube rupture should still be considered as a viable relief scenario). However, if the low-pressure fluid is liquid from a separator that has a small amount of vapor from flashing across a level control valve, the vapor pocket may collapse after the pressure has exceeded the fluid s bubble point. The bubble point will be at the separator pressure. Transient analysis will prediet a gradually inereasing pressure until the pressure reaches the bubble point. Then, the pressure will increase rapidly. For this ease, a transient analysis should be considered. 

Choking, or expansion of gas from a high pressure to a lower pressure, is generally required for control of gas flow rates. Choking is achieved by the use of a choke or a control valve. The pressure drop causes a decrease in the gas temperature, thus hydrates can form at the choke or control valve. The best way to calculate the temperature drop is to use a simulation computer program. The program will perform a flash calculation, internally balancing enthalpy. It will calculate the temperature downstream of the choke, which assures that the enthalpy of the mixture of gas and liquid upstream of the choke equals the enthalpy of the new mixture of more gas and less liquid downstream of the choke. 

When the weak aqua enters the ahsorher, it flashes or is expanded through a control valve from about a column pressure of 214.2 psig to the absorber pressure of 18 psig. At equilibrium for this 25.8% ammonia solution and at 18 psig, the temperature is 138°F (Figure 11-17), and the liquid enthalpy is 49 Btu/lb. 

The total flow required to the evaporator will be larger due to the flashing of ammonia across the control valve ahead of the evaporator. 

Let X = fraction (weight) ammonia vapor formed by flashing the 228.9 psia liquid at the control valve down to 38.5 psia. By heat balance per pound of ammonia ... 

Note that due to flashing and formation of a vapor-liquid mixture, the control valve is always placed as close to the inlet of the evaporator as possible. 

Pressure drop, chart, 102, 103, 111 Computer aided drafting, 17 Condensate, flashing flow, 135-142, 147 Charts, 142, 143 Control valve pressure drop, 90 Calculations, 90-96 Cost estimates, plant, 45-49 Accounting, 48... 

The careful selection and design of control valves is important good flow control must be achieved, whilst keeping the pressure drop as low as possible. The valve must also be sized to avoid the flashing of hot liquids and the super-critical flow of gases and vapours. Control valve sizing is discussed by Chaflin (1974). 

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. 

More detailed equations are available in publications of the control-valve manufacturers (for example, the Masonielan Handbook for Control Valve Sizing, Dresser Industries, 6th edition, 1977) that handle flows of gases, flashing liquids, and critical flows with either English or SI units. 

Even after we have made these two unlikely assumptions, the height of hot water in the draw-off sump must still be 56 in above the center-line of the draw-off nozzle. If not, the water would begin flashing to steam, as it experienced a pressure drop of 2 psi, flowing across the control valve. The evolved steam would then choke the water flow, reducing the pressure drop across the control valve until the pressure drop equaled the depth (or head) of water in the draw-off sump. 

Referring to Fig. 11.7, the pressure head of water above the control valve B is an extra 2 psi, or 56 in of water. Thus, even if the control valve B loses 2 psi of pressure, the water will not flash to steam. And this is true even when the sump is almost empty. 

The noise sources in control valves include mechanical vibration (usually below 100 dBA) hydrodynamic noise caused by liquid turbulence, cavitation, or flashing (usually below 110 dBA) and aerodynamic noise (can reach 150 dBA). In control valve design, aerodynamic noise can be a major problem. Aerodynamic noise generation, in general, is a function of mass flow rate and the pressure ratio (p /pf) across the valve. The point at which sonic speed is reached in the valve vena contracta is a function of the valve design. 

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