止回阀是自动运行的装置。他是利用流体速度压力来开启和保持在开启的位置，并利用重力的作用来关闭。阀座密封负载和组合的紧密性是与返回压力密切相关。如果流体流速压力无法将阀门支撑在一个较大的开启度并保持在稳定的开启位置，则阀瓣和相关的运动部件可能会处于一种持续振动的状态。为了避免出现运动部件的过早磨损、噪音或振动运行，就要根据流体状态选择止回阀的通径。能够将止回阀支撑在敞开的、稳定的开启状态所需的最小流体速度，需要通过大量的测试分析和下列公式进行计算得出：
Check valves are automatically actuated. They
are opened and sustained in the open position by the force of velocity
pressure, and closed by the force of gravity. Seating load and resultant
tightness is dependent upon back pressure. The disc and associated moving
parts may be in a constant state of movement if the velocity pressure
is not sufficient to hold the valve in a wide open and stable position.
Premature wear and noisy operation or vibration of the moving parts can
be avoided by selecting the size of check valve on the basis of flow conditions.
The minimum velocity required to hold a swing check valve in the wide
open and stable position has been developed by analysis of extensive test
data and is expressed by the formula:
v = 60 
v值表示流速，单位为英尺/秒； 是流体比容积，单位为立方英尺/磅。通过这种方式确定的止回阀通径，可能会出现所确定的止回阀通径小于管道通径，因此在安装时，可能需要安装减缩管。所产生的压降不能大于那些处于部分开启状态的通径较大的阀门，这样将大大延长阀门的使用寿命。这当然还带来额外的好处，那就是通径更小的阀门，成本也相应更低。
The value for v is equal
to flow in feet per second and
is the specific volume of fluid in cubic feet per pound. Sizing swing
check valves on this basis may often result in the use of valves that
are smaller than the pipe in which they are used, necessitating the use
of reducers for installation. The pressure drop will be no greater than
that of the larger valve that is only partially open, and valve life will
be greatly extended. The added bonus, of course, is the lower cost of
the smaller valve.
旋启式止回阀的阀座表面不存在磨损或刮伤的倾向，因为阀瓣与平面阀座相适应，不会在关闭时产生摩擦接触。
There is no tendency for the seating surfaces
of swing check valves to gall or score, because the disc meets the flat
seat squarely without rubbing contact upon closing.
如果用户需要，美国克雷恩旋启式止回阀可以配置外置杠杆和调节压铁。通过安装杠杆和压铁，有助于阀瓣的关闭，当流体停止流动，阀门的关闭速度将更快，从而将流体倒流、波动和冲击降到最小程度。安装的杠杆和压铁能够平衡阀瓣的重量，这样阀门对于比较低的流进速度将会更加灵敏。
Crane cast steel swing check valves can be furnished
with outside lever and adjustable weight when so ordered. With the lever
and weight mounted so that the weight assists the disc in closing, the
valve closes more rapidly when flow stops, thus minimizing reversal of
flow and resultant surge and shock. With the lever and weight mounted
to balance the weight of the disc, the valve becomes more sensitive to
low inlet velocities.
旋启式止回阀用于防止水平管路或垂直管路的流体倒流。在垂直管路，或者水平到垂直之间的任何角度的管路，止回阀只能用在流向朝上的管路上。
Swing check valves are used to prevent reversal
of flow in horizontal or vertical pipe lines. In vertical lines, or for
any angle from horizontal to vertical, they can be used for upward flow
only.
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