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Motherwell Control Systems pressure and vacuum relief valves provide relief of excess pressure or vacuum in a storage tank to prevent damage to the structure. Choosing the correct valve size is therefore of prime importance. Valves can be sized by Motherwell Control Systems through a computer programme, from flow curves or by the use of empirical formulae.

Venting of atmospheric and low-pressure storage tanks is covered by the API standard 2000. When determining the size of valves for normal and emergency venting the most common causes of overpressure and/or vacuum relief to be considered are as follows:

  • Product Emptying and Filling
  • Atmospheric Temperature Changes
  • Exposure to Fire

Emptying and Filling
When a tank is being filled, the space above the liquid level is compressed, causing an increase in internal pressure. Also a vacuum is created in internal pressure. Similarly, a vacuum is created within the tank when it is being emptied. At the pressure and vacuum relief valve, inbreathing results from the outflow of liquid and outbreathing results from the inflow of liquid.

Atmospheric Temperature Changes
A pressure and vacuum relief valve allows tank breathing during atmospheric temperature changes. A decrease in temperature changes, will cause contraction or condensation of vapours, which may result in breathing. Outbreathing may similarly be produced as a result of an increase in atmospheric temperature.

Exposure to Fire
In the event of a fire in the vicinity of the tank a significant expansion of the vapours and evaporation of liquid can occur. This will require a means of providing the necessary emergency relief outbreathing capacity.

If the pressure and vacuum relief valve size is not known this can be calculated by Motherwell Control Systems based on information on the required valve flowrate and pressures, or based on details of the tank.

To provide accurate pressure and vacuum relief valve sizes please submit the questionnaire, which lists some of the data required to correctly size a valve.

Click here to go to the Questionnaire