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