What is Pressure vacuum or PV valve , its working, principle and purpose ?

Pressure vacuum valve or PV valve in the ventilation system will prevent either over or under pressure. They are set usually so that tank pressure of about 0.14 bar will lift the main valve (The smaller valve will lift along with it) and release excess pressure. The vapour passes to atmosphere through a gauze flame trap. Drop in tank pressure compared with that of the outside atmosphere will make the small valve open downwards to equalize internal pressure with that outside

Pressure vacuum valve or PV valve can relieve moderate changes in tank pressure due to variations in temperature and vapour quantity. A drop towards vacuum conditions as the result of the condensation of steam will also be handled by the valve. Rapid pressure rise due to an explosion would not be relieved.

In other words pressure vacuum valve can be explained as:

Pressure vacuum valve or PV valve can relieve moderate changes in tank pressure due to variations in temperature and vapour quantity. A drop towards vacuum conditions as the result of the condensation of steam will also be handled by the valve. Rapid pressure rise due to an explosion would not be relieved.

The fast rate at which a tank is filled while loading produces a very rapid expulsion of the previous contents (vapour and inert gas). The pressure vacuum valve is not designed as a filling vent and neither should the tank hatch be left open. The latter method of venting can cause an accumulation of flammable vapours at deck level. Tanks should be vented while filling, through mast head vents or through special high velocity vent.

 The high velocity pressure / vacuum valve is a device to prevent the passage of flame consisting of a mechanical valve which adjusts the opening available for flow in accordance with the pressure at the inlet of the valve in such that the efflux velocity cannot be less than 30 m/s (98 ft. /sec) under all flow rates and the actual condition of installation.

Principle Of Operation

• Pressure vacuum relief valves have weight-loaded or spring-loaded pallets. Flow through the valve is controlled by the weight of the pallet or the spring force acting on the pallet to keep the device closed.
• Once the pressure or vacuum in the tank reaches the pallet closing force, the pallet will start to lift off the seat and allow flow through the valve.
• Due to its air-cushioned sealing technology, valves prevent emission losses until very close to the set pressure, and prevents air intake until very close to the set vacuum.
• The geometry of the valves has been developed in order to optimise overall performance in terms of high flow capacity, set pressure, sealing and re-sealing.
• The modular nature of valves means they can be tailored to meet specific customer requirements: whether this is pressure or vacuum-protection only; or vapours and gases either piped-away or vented to atmosphere.
• A weather hood protects the pressure vent port and a mesh screen is fitted to prevent intrusion of foreign matter into the valve

Precautions  regarding high-velocity valves.

A pressure vacuum (PV) valve is designed to release and/or let in pressure to protect the cargo tank from exploding or imploding due to too high or too low pressure in the tank. See picture of PV-valve on previous page. The PV-valve must be looked after due to the possibility that they may be clogged from freezing vapours, polymer build-up or icing up in adverse weather conditions, always be sure that the PV-valve is working before commence loading/discharging.