pumps demystified

Dear CAP:

You want different pumps for different applications. The environment where the motor will be used should determine whether to use an open drip proof, totally enclosed fan cooled (TEFC), explosion proof, or wash-down duty motor.

Any pump that would be used in a biodiesel application falls into one of two categories: positive displacement or rotodynamic.

Centrifugal

The only rotodynamic pump worth mentioning here is the centrifugal pump. The inexpensive Clearwater pumps that many homebrewers use on their Appleseeds are centrifugal pumps.

Centrifugal pumps are a poor choice for high viscosity (thick) liquids like cold oil or glycerin. They are a crummy choice for applications were self-priming (ability of pump to clear air from the supply piping and let liquid in) is necessary. They are a good choice when the supply lines are flooded with liquid at time of startup (read: effective self-priming centrifugal pumps are a myth) and work well with thinner liquids like water, methanol, or warm biodiesel. Centrifugal pumps also provide agitation as the fluid passes through the impeller and are capable to high flow rates. We use centrifugal pumps at Piedmont on our wash water applications and the finished product loading terminal.

There are a number of positive displacement pumps that work well in biodiesel applications because they are superior at handling viscous (thick) fluids and self-priming. The simple operating principle is for the pump to trap a portion of the incoming fluid and then force it into the discharge pipe.

Gear

Rotary gear pumps are a good choice for viscous fluids and in applications where self-priming is essential. Another advantage is that gear pumps can be run in either direction. We use one on a pump cart (one of our best ideas) that I put together for emergency and odd-maintenance operations.

Diaphragm

Diaphragm pumps are great self-primers but also tend towards lower flow rates. They are commonly run off compressed air which can be nice if sparking or electric arcing is a concern. We use a stainless steel diaphragm pump operating off compressed air to meter methoxide into our reactor. Accurate metering is possible because we know exactly how much methoxide the diaphragm displaces every cycle.

The Tuthill/FilRite fuel transfer pumps found on many farm tanks and biodiesel coop tanks are vane pumps.

This is by far the most widely used type of pump at Piemdont Biofuels. Our retail dispensers, reactors, and bulk oil, glycerin, and methanol storage tanks are all outfitted with vane pumps. They combine all the benefits of positive displacement pumps with good flow rates.

Power

Consider how your pump will be powered. Will it be pedal powered like a bicycle or hand powered like a fork and knife? Human power is great for high accuracy over a short period of time but if your application requires 12 hours of straight fluid transfer it's a poor choice.

Combustion engines can be used to drive motors and power take-offs (PTOs) from truck, tractor, or equipment transmissions are often used to drive pumps. Many pumps are powered by pneumatic or hydraulic sources. Personally, I prefer running my motors off good vibrations the way hippies run their tour buses.