How do you calculate the size of a Petrol/Oil Separator needed for a site?

The last in our customer questions regarding Petrol/Oil Interceptors/Separators is; How do you calculate the size of a Petrol/Oil Separator needed for a site?

Firstly you need to determine if you require a Bypass Interceptor or a Full Retention Interceptor; this is answered in a previous Customer Question answer; What are the different types of Petrol/Oil Interceptor (Separator) and what is the difference between them?  Secondly you need to determine if you require a Class 1 or Class 2 Interceptor; this is also answered in our last customer question answer; What is the difference between a Class 1 & Class 2 Separator?

After selecting the interceptor size and type, the correct size of interceptor is based on the flow into it based on rainfall onto a given surface area, this is known as nominal size or NS.

Each interceptor is allocated a nominal size based on EN858; the European Standard for Interceptor Design and Sizing.  This will indicate the nominal flow that the interceptor is sized for e.g. 4 l/s.  A full retention interceptor with a capacity of 4 l/s would be designated NS04 while a By-Pass interceptor with the same capacity would be designated NSB04.

(Please note Petrol/Oil Separator and interceptor mean the same thing)

Example 1

An interceptor required for a 25,000 m2 car park discharging into a local authority surface water sewer.

As there is a low chance of oil spillage a Bypass Interceptor would be most appropriate.

As discharge is to a surface water sewer a Class 1 Interceptor, which is designed to achieve a concentration of 5 mg/l of petrol/oil at discharge under normal conditions and includes a coalescer and a closure mechanism is most appropriate. A Class 1 Interceptor is usually required for discharge to any surface water.

Based on EN858 a drainage are of 25,000 m2 gives a nominal size (NS) of 45 l/s. 

So in this case BMS would select a BMS PB5 Class NSB 47 l/s Bypass Interceptor, which is the closest available in our range.

Example 2

An interceptor required for a 1,700 m2 fuelling area discharging direct to surface water.

Due to the high risk or fuel spillage a Full Retention Interceptor designed to contain the full contents of a full spill is required.

As discharge is to a surface water a Class 1 Interceptor, which is designed to achieve a concentration of 5 mg/l of petrol/oil at discharge under normal conditions and includes a coalescer and a closure mechanism is most appropriate. A Class 1 Interceptor is usually required for discharge to any surface water.

Based on EN858 a drainage area of 2,600 m2 gives a nominal size (NS) of 47 l/s. 

So in this case BMS would select a BMS PB9 Class 2 NS 47 l/s Full Retention Interceptor, which is the closest available in our range.  Please note that the Nominal Size of a Full Retention Interceptor can also be denoted as NSF e.g. NSF; 47 l/s.

BMS manufacture produce a full range of 'plug & play' interceptors/separators from 1,000 - 100,000 litres manufactured in corrosion and rot proof GRP under ISO 9001:2008 to European Standards and are Irelands leading producer.

BMS have introduced a full set of Petrol Interceptor/Holding Tank Sizing & Selection Charts to help you easily size and select an Interceptor/Holding Tank suitable for your requirements. They can be downloaded at this link.

If you have any other questions or queries please contact us.