Attività

If a valve may be required to remain in place closed on removal of the pipe on one side for a temporary operation, it must be flanged for bolting to a pipe flange on the other side. ‘Wafer’ butterfly valves whose bodies are sandwiched between pipe flanges do not achieve this. Use of such valves for isolation of air valves allows maintenance to be carried out on the air valve in situ with the pipeline in service but does not allow removal and replacement of the air valve under pressure. Since replacement of air valves is likely to be cheaper than in situ refurbishment, flanged isolating valves are preferred in such situations.

The butterfly valve is a rotary valve in which a disk-shaped seating element is rotated 90° to open or close the flow passage. They are used in throttling service, particularly where large-size valves with automatic actuators are required. Butterfly valves cannot be used where a nonobstructed, full opening is needed. They offer a size and weight advantage over plug and ball valves.

Conventional butterfly valves are used mainly in low-pressure water service and throttling applications. The seats, disk, and shaft are in the same plane. The seat is obtained by an interference fit between the disk and resilient (flexible) liner. This type of fit is shown in Figure 4.64. The tightness of the seat is limited by the operating torque of valve and the seal between the shaft and the liner. The sealing characteristics of this valve are poor and leakage usually occurs.

The high-performance butterfly valve provides good sealing characteristics and a tight shutoff. The disk is essentially an off-center slice of a ball, and the seating mechanism of this valve is similar to that of a ball valve. The disk and seats of this valve are offset from the shaft and shaft sealing in this valve is not critical. Many valves offer a primary seat made of a resilient material and a secondary metal-to-metal seal making them “fire-safe.” High-performance butterfly valves are available in pressure classes as high as ANSI 900 and can be used in applications requiring tight shutoff.

The gate valve has a unique body style unlike the other valves we have discussed. The butterfly uses a circular plate or wafer operated by a wrench to control flow. A 90° turn of the wrench moves the wafer from a fully open position to a fully closed position. The wafer remains in the stream of flow and rotates around a shaft connected to the wrench. As the valve is being closed, the wafer rotates to become perpendicular to the direction of flow and acts as a dam to reduce or stop the flow.

Traditional butterfly valves now work at high pressure drops across the disc which can be both metallic and “soft”. Upper and lower temperature limits are the same, by and large, as those for globe valves, depending on duty and material of construction. The butterfly construction is especially suitable for high temperatures. Bodies can be fabricated from bar and plate and the seals can be mounted on cooling extensions away from the main flow. 

Butterfly valves can be used as a control valve and also as a shut-off valve, as discussed in Chapter 3, Section 3.3.3, against high pressure drops of regularly up to 415 barg. Depending upon the materials of construction and the seat design a butterfly control valve may have very limited shut-off pressure drops. Some 100 barg valves are only rated for 4 barg shut-off differential.

Generally, butterfly valves are used for the inlet control and bypass valves. They are inexpensive to manufacture, and their actuators are able to operate in accordance with the requisite response times. Butterfly valves do, however, have the disadvantage of a tightly curved characteristic.

Additional non-linearities arise from the fact that valves of different nominal sizes are operated in sequence. An initial improvement in the control response was achieved in that the steady-state duty point characteristics for operation with and without the expander were stored in function generators in the controller. Depending on the operational state, the output of the process controller (regenerator pressure, or differential pressure, between the regenerator and the reactor) is applied to one or the other of these characteristics. In the event of an expander trip, the system immediately switches from one characteristic to the other. This results in linearization of the characteristic profile, so that the process controller is able to operate independent of the duty point concerned and independent of the operating mode (i.e., with or without the expander). This switching between characteristics in the event of an emergency trip also ensures that the bypass valves are always driven at maximum actuating speed to their new steady-state position in accordance with the prevailing operating conditions. All this is performed independent of the prevailing duty point (i.e., whether the system is operating at partial load or at overload).

In the industry, globe valves, which are commonly used to precise control the flow rate along with opening and closing flow in pipes, are technically and economically limited in valve size due to structural instability related to complex internal flow passage. Butterfly valves, on the other hand, have advantages such as low weight and low manufacturing costs, but it is difficult to control the flow rate at an opening angle of 60°or higher and flow is unstable in the case of butterfly valve.