pressure vacuum valve is used in the ventilation system to keep the over or under pressure of the tank within the approved limits. So that under all the flow rates, the efflux velocity cannot be below the 30 m/s (98 ft. /Sec) and kept under the actual installation conditions.

Sufficient damage or rupture may result due to a moderate i.e. 0.24 bar pressure acting on the large surfaces of the liquid cargo tanks. If the pressure within the tank falls below the pre-set threshold, the valve of the vacuum opens to pass air through the flame trap into the tank.

Two release valves open in case of overpressure, to ensure the upward venting at high velocity and helps to avoid gas concentration on the deck.

The P/V valve makes the flow of small volumes of air, inert gas mixtures, or vapor easily, which is caused by the thermal variations in the cargo tank. Due to the vapor quantity and temperature variations, the pressure vacuum valve relieves the temperate changes in the tank pressure.

The PV valve also handles the steam condensation, which results in the drop towards the vacuum conditions. The rapid expulsion of inert gas and vapor contents are produced while loading tank at a fast rate. The PV valve is not designed as a filling vent, and also the tank hatch should be left open.

This venting method may cause an increase in the flammable vapors at the deck level. While filling, the tanks must be vented through the pole head or special high-velocity vent.

Principle operation of PV Valve

  • PV valves have spring-loaded or weight-loaded pallets. The pallet valve or force of the spring acting on the pallet controls the flow through the PV valve and keep the device locked as well.
  • Once the tank pressure and pallet closing force are equal, the pallet begins to elevate the seat and let the flow through the valve.
  • The air-cushioned sealing in the valve fabrication avoids the emission losses until its very near to the set pressure level as well as block the air intake when it approaches the set vacuum.
  • To optimize the overall performance of the valve in terms of set pressure, re-sealing, sealing, and high flow capacity, the valve geometry has been developed.
  • The modular valve nature allows meeting the customized requirements such as to use it as a vacuum or pressure protection only or gases and vapors either vent to atmosphere or piped-away.
  • The pressure vent port is protected by the weather hood port, and to stop the invasion of external content into the valve, a mash screen fits in it.

The normal IG deck pressure kept in the liquid cargo tanks is 700-800mm Water Column. The low-pressure alarm is set at 200mm Water Column, whereas 100mm Water Column is the low-pressure trip. At this level, the cargo pumps may trip off.

The lift setting of valve pressure is operated at 1400mm Water Column, and the vacuum valve is set at -250mm of water Column. Normally, the cargo tank is tested to the positive pressure of 2500mm of water Column and -750mm of Water Column to negative pressure.