Check Valve

1 Typical Usage
Check valves are used to block the pipe flow in one direction while permitting flow in the other direction. A typical application of a check valve is at the discharge of a pump. If the pump is idle on stand-by the check valve prevents reverse flow though this pump; the moment the pump start the flow can lift the check valve and go in the downstream direction. 

2 Primary Function

The primary function of a check valve is to prevent flow reversal. Check valves pass fluid freely in one direction and, if pressure reverses, close to stop flow in the other direction. Therefore, the principal force on the check valve is a water or steam hammer transient load.

3 Application Objectives

The major engineering concern in design and selection of check valves is to achieve, at normal flow condition, a low pressure drop across the valve, a reasonable wear rate on the valve element, and a stable operating mode. The design and selection of the check valve should also include the attainment of an acceptably low transient load and a tolerable leakage rate during and after valve closure.

Globe Valves

1 General Characteristics

The globe valve controls flow by raising or lowering a circular disc on a seat. Globe valve are normally manufactured in sizes from 3/8 inch to 16 inch. Globe valves larger than 16 inch can be obtained at a premium price. Normally flow direction is from under the seat, therefore, when the valve is closed the full line pressure is exerted on the disc and stem. On large valves the force needed to keep the valve closed and leaktight are great, hence, the size limitation. For the same reason, valve actuators tend to be larger than those for gate valves. Also a, self-locking gear is needed to hold the valve closed. The globe valve should be used primarily for throttling service at disc positions greater than 30% from the fully closed portion as a block valve. In vacuum service, globe valves are frequently installed opposite to the direction specified for pressure service.

2 Body Configurations

Globe valves are manufactures in three basic body configurations  :
a. Tee pattern
b. Wye pattern
c. Angle pattern (Angle Valve)

3 Disc Flow Characteristics

Discs may be specified with several contours depending on desired flow characteristics, such as :
a. Quick opening
b. Semi-throttle
c. Linear
d. Equal percentage

 Most globe valves are furnished with quick-opening or semi-throttling discs and are satisfactory for on-off service or when precise flow control is not required over the whole travel range of the valve. If control of flow is required over the complete travel range, other discs such as “linear” or “equal percentage” or, for small valves, needle plug are available.

4 Stem/Yoke Configurations

Globe valve stem/yokes are typically manufactured in two different combinations :
a. Outside screw and yoke with rising stem
b. Inside screw rising stem

5 “Packless” Designs

Most globe valves have a stuffing box as the stem seal, however, “packless” valves are also available. These design use a diaphragm or bellows to provide the seal and also have backup packing in case of diaphragm or bellows failure. They are used to prevent leakage to the environment and in vacuum service to prevent atmospheric leakage.

6 Backseats

Back seats may or may not be specified for globe valves; if specified, they are energized only when the valve is fully open. It is normal practice to specify a backseat; however, extreme caution should be exercised in using backseat.

7 Disc Guiding

Stem and disc to body alignment can be accomplished by “top guiding” or “body guiding”, generally in lower pressure valves with tee pattern. “Bottom guiding” is not recommended as these design can have a relatively short service life and may be prone to breakage. Good alignment of the disc to the seat is necessary for effective sealing.

8 Seats

Globe valve seats can be integral, replaceable, hard faced, soft faced, resilient, etc., as required. The seat material can be selected to suit the application.

9 Limitations

Small (2 inch or smaller) globe valves may be used as stop valves because the force under the disc is within the capability of the stem threads to keep the valve closed. For valves that usually remain partially opened, deep stuffing boxes are required, and the disc should be guided along the full stem travel.
In general, the maximum differential pressure across globe valves should not exceed 20% of the upsteam line pressure and never exceed 200 psi, unless a special trim and disc design has been provided by the valves manufacturer. Prior to using a given design, the valve manufacturer should be consulted regarding the valve design capability for these parameters.
Small globe valves can be used as piping or equipment high point vent or low point drain connections as well as root valves for instrument pressure connections. Globe valves with special trim are used for blowoff or blowdown system applications.

10 Body Configurations

The various body configurations of globe valves should be used and applied as follows :
a. Tee Pattern : To be used when severe throttling is required as in a by pass line around a control valve where pressure drop is not a concern. This pattern has the lowest flow coefficient (Cv).
b. Wye Pattern : To be used when valve is wide open or throttling during seasonal or startup operations. This valve type has very little resistance to flow and can be “cracked open” for long periods without severe erosion.
c. Angle Pattern : Similar in application to the wye pattern globe with a slightly lower Cv. A particularly good selection for systems that have periods of pulsating flow because this valve configuration adequately handles the “slugging” effect inherent with this type of flow. The “angle” valve, if appropriately located, can eliminate a 90 degree elbow.