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INSTALLATION/SERVICE RECOMMENDATIONS

GATE VALVES GLOBE VALVES

The flow through gate valves may be bi-directional. Theremay be exceptions to this if bypass or equalizing piping is welded to the valve body or if a pressure relief hole is drilled in one side of the wedge. Check your piping layout drawing to ensure correct position and direction of flow. Gate valves should be installed and welded into the pipe-line with the wedge or disc in the fully closed position. If the valve is left open or partially open, it could distort and leak during operation. Leaving the valve in a fully closed position helps prevent weld spatter from falling directlyonto the mating faces of the seats and wedge.The preferred orientation of a gate valve is stem vertical.The valve may be installed in other orientations, but any deviation from vertical is a compromise. Installation upside down is not recommended because of possible dirt build-up in the bonnet. Consult the Velan Engineering Department during the quotation review process. Hard- facing of guides may be required when valves over 12” (300 mm) are tilted beyond 45° from the vertical position. Globe valves are usually installed with the inlet below the valve seat. This must be checked carefully to prevent incorrect installation. If throttling service is particularly severe, Velan recommends that the valve be installed so that the flow enters over the top of the seat and goes down through it. This maintains the valve in a more stable condition, the amount of wear is minimized and there is less external noise. Valve operation also becomes easier because less torque is required to close the valve.However, it should be noted that the packing is underconstant load. Regular style globe valves are suitable for moderate throttling applications. As a general rule, an adequately sized globe valve (i.e. with pipe velocity between 15 to 25 ft/sec (4.5 to 7.5 m/s) for water and 200 to 300 ft/sec (60 to 90 m/s) for steam) should not be throttled down below 35% of its maximum full open Cvcapacity (approximately 20% of full stroke). Harsh throttling, below 35% of full Cvcapacity, willrequire analysis by Velan Engineering to determine suitability under possible cavitation, flashing, noise and vibration.Globe valves should be installed and welded with thedisc in a fully closed position to prevent damage to the valve during installation. Leaving the disc in a fully closed position firmly closed also prevents weld spatter from falling directly onto the mating faces of the seat and disc.For both the fully back seated and fully closed position, it is recommended that the stem be backed off NOTE: Gate valves should not be used for throttling to control the flow, they are normally fully open or fully closed. If left in partially open position, severe damage to body seats, wedge, stem AND guide rails may result.
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8 to
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4 PRECAUTION: turn to prevent the stem from “growing”, bending orcausing thermal binding. Taking this action will not affect valve shut off. Allow time for welding to cool before stroking the valve for the first time.When required, it is recommended that local P.W.H.T.be performed with the valve in the closed position. Pressure seal design with live-loaded bonnet:

PRESSURE SEAL GASKETTORQUES

The pressure seal bonnet bolt tightness must be rechecked after operating test pressure and temper- ature has been achieved. Without live-loading, bolt tightness will be lost when the system loses pressure. A live-loaded bonnet bolt may also lose some boltload, due to vibration in transit or bolt creep. It isadvisable that live-loaded bonnets be checked for bolttorque at operating pressure and temperature. Once torqued at operating pressure, no future tightening ofbonnet bolting will be necessary, unless the valve hasbeen dismantled, in which case, the bolts must be torqued to appropriate values and bolttightness, rechecked after operating test pressure and temperature has been achieved. Pressure seal design without live-loaded bonnet (no spring washers): CAUTION: If the pressure seal gasket is not pulled upconcentric to the body/bonnet top-bore axis, underlower pressure, the gasket may not be properlyseated. Consequently, leakage may occur. Moreover, non-uniform contact may be the sourceof leaks in the future. Therefore, when possible, backseat the valve fully when tightening up the pressure seal gasket bolts.

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