Another tip for making your oil water separation process easier. A few weeks ago we wrote about the first rule of thumb, get the oil out or off the water as soon as possible.
The following post will address emulsions and how they can effect and even fool an oil water separator.
If you eventually want to separate oil from water, WATCH OUT FOR HOW YOU PUMP IT. Example: Often engineering firms and consultants specify use of an oil water separator for treating oily water coming from elevator sumps. Makes sense right ? Well it almost makes sense...kinda'
An oil water separator (OWS) is a flow thru device (USUALLY GRAVITY FLOW) designed for removing free unemulsified oils from a moving stream of water. The OWS is designed based on the differential in the specific gravity between oil and water (Stokes law) to do it's separation. Most oils are lighter than water so they float on the surface of the water.
Most elevator sumps contain and use a submersible sump pump for pumping out any water that can seep or leak in. These are usually centrifugal off the shelf very reliable pumps but the problem can be that they are high shear type pumps that act like a mixmaster and chop the oil particles into micro oil particles which are so small they don't have enough buoyancy to rise thru the water column. Problem is----now you've created a mechanical emulsion that an oil water separator is designed to handle. An emulsion can fool an oil water separator.......avoid whenever possible.
ALL BETS ARE OFF FOR RUNNING IT THROUGH AN OIL WATER SEPARATOR if the oily water has been thru a centrifugal or other high shear pump. There are instances where we can put pumped oily water thru an OWS but we have to take a hard look at all the parameters of this type application and ALWAYS, it's going to take an much bigger separator that will provide for lots more dwell or residence time in the OWS to effect separation. It doesn't matter if the oily water was pumped yesterday or last week or last month, now you've created an emulsion that's a game changer for easy separation. Pumping the oily water gently, as with a positive displacement (PD) pump may let you work with an oversized OWS but it's a tricky formula to work out.
We use a general rule of thumb of 20% that can work some of the time depending on all conditions. That is if the stuff has been gently pumped you can feed it to an OWS at 20% of the OWS's gravity flow rate, but this is not a hard and fast or constant rule. There are some work arounds for the pumping of oily water, but whenever possible DON'T PUMP IT if you want to separate it later. Why deal with a problem you can avoid to begin, with thru good design.