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Block and Bleed Valves selection

asked Jun 4, 2015 in Engineering by TPham (4,980 points)

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answered Jun 4, 2015 by TPham (4,980 points)

Block Valves

  • Double gate valves, or single valve alternatives with double block capability, should be provided.  Alternatively, a single gate valve may be used when justified by:
    • A risk assessment conducted by the Owner for the specific application or
    • Successful experience with the specific service involved.
  • In such cases, the double block and bleed closure test may be waived if approved.
  • Use of other single or double block valve types as an alternative to the use of gate valves for block and bleed applications should also be approved.  Single block valve alternatives may include special gate, ball, and plug valve designs.  These valves should be closure tested for double block and bleed per API STD 598.
  • Where a single valve is used, the valve should be a special "double block and bleed" type gate, or alternatively, a soft seated plug or ball type.  A closure test for double block and bleed per API STD 598 is required, with the test pressure equal to the valve design pressure. The use of a single resilient seated butterfly valve is permitted where zero leakage has been demonstrated by past experience or special testing and where leak detection facilities are provided.  In this case a body bleed is not required.
  • Where double gate valves are used:
    • All seat rings should be seal welded to the valve body.
    • The backseat bushing should be hardfaced with CoCr Alloy.  Additionally, wedge guides should be hardfaced with CoCr Alloy for valves cycling ten or more times per year.
    • Both wedge gate and seats should be hardfaced with CoCr Alloy for non-13 Cr trim materials.
  • Double block valves should be used when a single block valve may be rendered inoperable due to auto-refrigeration of a volatile liquid.  The valves should be separated by at least six inches of pipe.  A bleeder is not required in this case. A backup valve will allow for fluid control even if one valve is inoperable.
  • When double valves are required, two valves separated by a short spool piece fitted with a bleeder should be provided; alternatively, two valves bolted together with an intervening bleeder installed (on either valve) in position C or D per ASME B16.34 is acceptable.  No connection other than the bleeder should be made between the valves. Defines the requirement that the two valves are reasonably close to each other and easy to identify/access as a double block pair, and not separated by a significant length of piping.
  • Double block valves should be provided to isolate equipment that has a spare or may be bypassed temporarily if it is probable a single block valve will become corroded, eroded, or fouled such that the installation of a blind would be difficult due to excessive leakage.
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answered Jun 4, 2015 by TPham (4,980 points)

Bleed Valves

A single bleed valve, plugged, capped, blinded, or piped to a safe location when specified, should be provided for all required applications (Plugging, capping, blinding, or piped to a safe location is required for personnel protection).  Bleed valves should be NPS 3/4 minimum and should have threaded outlets.  Bleed valves should be located as follows:

  • Bleed valve is required to safely depressure the system, check for leakage, and/or direct leakage to closed drain system.
  • For pressure relieving (PR) devices, Bleed valve is required to safely depressure the system, check for leakage, and/or direct leakage to closed drain system:
    • Between the PR device and the inlet block valve.
    • Between the PR device and the outlet block valve unless the PR device discharges to atmosphere.
    • For all shell-and-tube and double pipe heat exchangers, when a manually operated bleeder valve is used for overpressure protection against liquid thermal expansion, the valve should be installed on the cooler side.
  • When a body bleed is required, the bleed valve should be located on the block valve body or bonnet (for gate valves:  position G, H, I, J, or K per ASME B16.34, Figure 1) for valves NPS 3 (75 mm) and larger.  Where it is not feasible to provide a bleed (due to size or other constraints), two block valves with an intervening bleeder should be provided.
  • Between the block valve and the associated equipment item for all onstream equipment isolation applications, a drain valve already specified in this location, and not piped to a closed drain system, is sufficient to meet this requirement.
  • Between the block valve and isolation blind for onstream isolation of equipment in dangerous material services, unless the block valve is equipped with a body bleed in position C or D on the blinding side of the valve, defines alternative bleed valve location to safely depressure the system, check for leakage, and/or direct leakage to closed drain system.
  • Bleed valves are not required for:
    • Single and double block valves at instrument takeoff connections.
    • Double block at vents, drains, flushing, and cleaning connections.
    • Where PR devices are installed for thermal liquid expansion protection.
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answered Jun 5, 2015 by TPham (4,980 points)

Here are the requirements for the following services and applications for Block and Bleed Valve:

  • Onstream equipment isolation (including instruments)
  • Product segregation to prevent contamination
  • Segregation for safety
  • Slurry service
  • General (other) service applications

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