Safety valves are fitted with several small openings on the valve body, each serving a distinct engineering purpose. These ports are not merely manufacturing artifacts; they are designed for specific operational functions. This note describes the three common ports – their locations, intended roles, and the proper handling during installation, inspection, and maintenance.
1. Bottom Drain Port (at the lowest point below the seat)
Location: At the lowest cavity point just below the valve seat sealing surface, usually closed with a threaded plug.
Function: To drain condensed water, residual process fluid, or solid debris that may accumulate inside the valve chamber. For steam or liquid services, this prevents corrosion of the spring, sticking of the disc, or obstruction of the moving parts.
Sizing reference: According to GB/T 12243, the port diameter shall be at least 6.5 mm for nominal sizes up to DN65, and at least 9.5 mm for larger sizes. If the valve body itself does not have such a port, an equivalent weep hole should be provided on the downstream piping.
Maintenance points:
During normal operation, the plug should remain tightened to avoid leakage.
Periodically (e.g., at each turnaround), open the plug to purge any accumulated liquid or solids, then re‑tighten it securely.
Ensure that painting, insulation, or external cladding does not cover or block this opening.
2. Upper Vent Port (only for bellows‑balanced safety valves)
Location: On the upper part of the bonnet (spring chamber), either on top or on the side.
Function: This port is provided exclusively for valves with a balanced bellows construction. It vents the bellows interior to the atmosphere, thereby eliminating the influence of back pressure on the set pressure. Additionally, if the bellows ruptures, the process medium will escape through this port, serving as a direct visual indication of bellows failure.
Important distinction: For ordinary non‑bellows valves, this position is typically sealed with a solid plug and requires no action. Only bellows‑equipped valves need this port open.
Maintenance points:
The valve is usually shipped with a temporary plastic dust cap on this port. This cap must be removed before commissioning. Leaving it in place creates a trapped volume inside the bellows, which can interfere with proper stroke and may even cause the bellows to distort under thermal expansion.
For flammable, toxic, or valuable media, this vent should be piped to a safe disposal point, and the open end fitted with a weather‑proof breather cap to keep out dirt.
If a continuous or intermittent flow of medium is observed from this port during service, it indicates that the bellows has failed – the valve should be taken out of service and replaced.
3. Side or Top Adjustment Port (for set‑pressure setting)
Location: On the side or top of the bonnet, accessible via a lock‑nut and an adjusting screw, typically sealed with a lead seal wire or a seal cap.
Function: This port provides access to the spring‑adjusting mechanism. By turning the screw, the spring pre‑compression is altered, thereby adjusting the valve's set pressure (opening pressure). Clockwise rotation increases the set pressure; counter‑clockwise decreases it.
Access restriction: Only authorised and qualified personnel are permitted to adjust this screw.
Maintenance points:
The lead seal must remain intact during normal service. No unauthorised person should break the seal or tamper with the screw.
Whenever the seal is broken for recalibration or overhaul, the set pressure must be re‑verified on a test bench, and a new lead seal applied after adjustment. The new set value should be recorded.
Routine inspections should only verify the integrity of the seal; no attempt to turn the screw is required.
General Reminder
These three ports are standard features of spring‑loaded safety valves, each with a clear role: drainage, back‑pressure equalisation and leak indication, and set‑pressure locking. They are not optional or redundant. During installation and maintenance, always check the valve nameplate and supplier documentation to confirm which ports are fitted and how they should be treated.
For the bottom drain, keep the plug closed except when purging.
For the bellows vent, remove the shipping cap and, if necessary, connect a discharge pipe.
For the adjustment port, respect the lead seal and leave adjustments to certified technicians.
By following these practical guidelines, the valve will maintain its designed performance and reliability without unnecessary complications.
