Other Bypass System Hardware

Orifices In high pressure applications, the system often does not provide adequate pressure in the bypass line to prevent cavitation or flashing. Either of these conditions is undesirable because it can cause damage to both valves and the pipe system or cause a reduction in flow below the minimum desired, jeopardizing the pump protection system. All pressure-reducing valves will experience a velocity-induced recovery effect that will limit the amount of pressure drop a valve can take and cause a reduction in flow capacity.

The requirement of backpressure is generic to all pressure-reducing applications. Pressure reduction even by multiple stage cascading can minimize the requirement; however, no valve design will redefine a fluid's physical properties. This becomes especially important in modulating systems. A fixed orifice will not provide the proper backpressure at all flow levels.

As the flow in the bypass line is reduced, the orifice becomes less effective. Proper system design should be used to optimize valve pressure reduction and consider all fluid dynamic effects downstream of any pressure-reducing device. When adequate backpressure is not available downstream of a pressure reducing valve, vapor bubbles will form in the zone just downstream of the valve last stage control surface. This zone is defined as the "Vena Contracta" and represents the point of highest fluid velocity and lowest pressure.

The potential for 1) damage to downstream piping components and 2) flow reduction exists from this point. When line pressure remains below the fluid vapor pressure, any existing bubbles will remain and expand as piping friction further reduces line pressure. This can be defined as "flashing condition" and is characterized by a polished appearance on affected surfaces. When the line pressure drops below the fluid vapor pressure and then recovers, any entrapped vapor bubbles will collapse (implode). This is defined as a "cavi-tating condition" and is characterized by a cinder-like appearance on affected surfaces. The resolution of either condition is best addressed by eliminating vapor formation. This can be assured by the provision of adequate back pressure through the use of a fixed or variable orifice.

Fixed Orifice Simple, easily replaced orifices that reduce the pressure are an effective way to reduce bypass head and provide adequate backpressure in bypass systems. Several stages may be necessary, however, to break down high-pressure drops without flashing. For calculations of flow through standard-shaped orifices, see Section 8.1 and 8.2.

Coefficients of discharge for oddly shaped multistage orifices are difficult to calculate. However, manufacturers of these specialties can furnish curves of delivery as a function of pressure.

Variable Orifice In modulating systems, a fixed orifice will not provide the proper backpressure over a wide flow range. A backpressure regulator (BPR) has a variable orifice with a spring-loaded plunger that is designed to open at a specified differential pressure. If flow and differential pressure increase, it opens further to maintain the differential pressure and backpressure constant. Figure 12 illustrates a typical BPR construction. Figure 13 shows a standard BPR installation. The BPR is normally located as close to the receiver vessel as possible so that the correct backpressure is maintained in the entire bypass line.

Valves, Piping, and Fittings For cold water at low pressure, a simple power-actuated globe-type bypass valve is often adequate. In modulating bypass systems, the bypass valve must resist throttling damage, particularly if the water is hot. Staging the pressure drop in the valve is the most common way to reduce or eliminate flashing and cavitation damage to the valve trim or body. Figure 14 illustrates a typical multi-stage pressure reducing valve (PRV).

Pressure is reduced in stages to ensure that the pressure never decreases below the fluid vapor pressure. This prevents cavitation and the resultant valve damage and noise. Figure 15 illustrates typical calculations for reducing pressure in sequential stages. Refer to Chapter 7 for detailed information regarding valve sizing and selection.

FIGURE 12 Construction of a typical backpressure regulator
FIGURE 13 Backpressure regulator installation

Pipe material like that used for the main discharge is adequate for nearly all of the bypass line. Near the orifice and control valve outlets, however, heavier wall or higher chrome content will lengthen life. Welded piping is common for high pressures. To prevent erosion, pipe fittings (especially elbows) should not immediately follow an orifice.

Flow Meters Flow rate is the variable that must be measured for most automatic bypass multi-component control systems. The meter may have any type of primary element that will produce an accurate signal at the process flow for which the bypass must be controlled.

A simple orifice meter or venturi tube is commonly used. The user must have the required straight upstream and downstream pipe lengths or use flow straighteners to obtain an accurate reading. The device must be properly sized to provide both accurate indication at relatively low process flows and satisfactory pressure drop at maximum process flow conditions.

Flow meters can be located either upstream or downstream of the pump. Meters located upstream are at lower internal pressure but are larger in diameter with larger flanges. In addition, pressure drop at rated flow is important to avoid low pump NPSH. Meters located downstream are normally smaller but must be rated for the maximum pump discharge pressure.

Controls Controls range in complexity from a simple pressure switch that stops a pump drive motor to modulating systems that meter flow and maintain a selected minimum flow through the pump. The control system may have to provide a deadband near the opening point and be sufficiently stable to hold bypass flow nearly constant in spite of erratic flow during upsets and startup.

For example, when the primary flow-measuring element is upstream of the bypass branch off, sufficient decease in flow will cause the bypass control to open in an "On-Off" bypass system. The main control valve, farther downstream, will then also open to maintain flow to the boiler or process. The primary element will sense the added flow and, in a simple bypass control system, close the bypass valve, initiating hunting in flow and valve action. To prevent this, the setpoint for bypass valve closing must be greater than twice the minimum bypass flow. If minimum flow is in the 30-50% range, this wastes energy during bypass. This control problem is avoided if a modulating system is installed.

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Survival Treasure

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