a) gate valves

Positive Features - Recommendations
  　　　1) Good choice for on-off service
  　　　2) Full flow-low pressure drop
  　　　3) Bidirectional

 Disadvantages
  　　　1) Not for throttling; use fully opened or fully closed
  　　　2) Metal-to-metal seating means not best choice for frequent operation. Bubble-tight seating should not be expected with metal-to-metal design.
  　　　3) Difficult to automate.

  Wedge and Seat Designs
      　　1) Solid wedge best choice for all building services. Modern foundry and machining controls enable the solid wedge to give good performance in a broad range of services.
  　　　2) Metal solid wedge seating against metal seat in body is the most common in building service piping. Rubber-coated solid wedge (resilient wedge) has become popular in fire protection and municipal cold water service. The resilient wedge design offers bubble-tight seating, but is limited to 160°F water service.

  Stem Designs
  　　　1) Rising stem-inside screw is the most common and preferred design for bronze multi-turn valves. In the fully open “back seated” position*, the stem threads are protected from the circulation of flow media. Because the stem rises as the valve is opened, clearance must be provided and stem orientation should
protect it from external damage. The stem position indicates to the observer if the valve is opened or closed.
*Repacking while valve is pressurized is not recommended.
  　　　2) Non-rising-inside screw stems are used in bronze and iron gate valves. While they are more compact than rising stem valves, the stem threads are always exposed to flowing media, allowing for possible erosion/corrosion. Open/close indication is not provided with this design.
      　　3) Rising stem-outside screw and yoke is preferred in iron multi-turn valves. threads are external to the valve, protecting them from flow media and enabling lubrication. Care should be taken for clearance as the stem rises so that threads are not damaged. The stem position provides indication that valve is opened or closed.

　Body-Bonnet Connection
1) Union bonnet is recommended as the best choice for building services piping for bronze multi-turn valves, although not all styles offer this design. The union bonnet offers strength and ease of assembly-disassembly.　　　
2) Threaded-in bonnet is common in lighter-duty services and smaller sizes of bronze multi-turn valves.
3) Bolted body-bonnet connection is the principal design used in iron multi-turn valves. This gasketed joint is strong and easy to assemble/disassemble.

b) Globe/Angle-globe valves

　　Positive Features - Recommendations
1) Recommended for throttling applications
2) Positive bubble-tight shutoff when equipped with resilient seating
3) Good for frequent operation

　　Disadvantages
1) Significant pressure drop due to flow path
2) More costly than alternate valves

　　Disc and Seat Designs
  　 　1) Resilient (soft) seat discs are preferred over metalto-metal except where temperatures, very close throttling or abrasive flow make metal alloy seating a better choice. TFE is the best resilient disc material for most services, although rubber’s softness gives good performance in cold water.
   2) Plug-type disc/seat, often with harder metal alloys, is best for very close throttling, when media temperature is beyond the range of TFE (500°F) or when abrasive flow is involved. Iron globe valves most commonly use semi-plug bronze disc/seat design.
3) Automatic steam stop-check angle-globe valves are best on medium-pressure steam service where the sliding action of the semi-plug disc assembly permits the valve to serve as a shut-off valve, a throttling valve and a check valve.