The ease of repairing sliding vane pumps should bring an end to the repair-or-replace argument.
By Chris Hordyk
There are philosophical choices that have sparked some of the great debates of our time: Star Wars or Star Trek? Thin crust or deep dish? Yanni or Laurel?
In the world of truck transport of liquid commodities such as motor fuels, heating oil and chemicals, the question that typically divides people into two distinct camps is “repair or replace?” Or more to the point, what is the best pumping technology with which to outfit your transport vehicles, one that can be repaired in the field, or one that should just be scrapped and replaced with a new model once it fails?
Surveying the Field
The operators of transport-truck fleets have a basic choice of three customary truck-mounted pump technologies: the sliding vane, along with external gear and centrifugal. Let’s take a closer look at all three technologies:
- External Gear: This pump technology uses the meshing of gears to facilitate the flow of liquid through the pump at consistent volumes, which makes them a positive displacement (PD) pumping technology. They will deliver a constant amount of liquid with each revolution of the gears, while their tight clearances and speed of rotation restrict any fluid from moving backward, or “slipping,” during their operation. Since the gears are rigid, the pumps create a smooth, pulse-free flow, but also one that can handle very high pumping pressures, especially those that are needed to transfer high-viscosity liquids.
However, the contact inherent in the meshing of the gears will cause them to wear over time. This gear wear will compromise volumetric consistency and increase the risk that product slip will occur as the pump ages, which will result in decreased productivity over time. External gear pumps are also generally inexpensive, though they do have a relatively high number of wear parts. This will prompt many users to run them to failure and then replace, rather than repair. The cost to replace the wear items is a substantial percentage of the entire cost of the pump and directs users to scrap the entire pump for common wear.
- Centrifugal: This technology uses the rotational energy created by an internal impeller to “throw” the liquid to the discharge port. Although less efficient than PD pumps (meaning they require more energy to perform the same amount of work) this method of operation produces a smooth, pulse-free flow. Conversely, to achieve this effect and the resulting operational benefits by design centrifugal pumps are more complex, which means higher downtime and maintenance costs when they fail. Centrifugal pumps also tend to have a steeper purchase price, which can put the operator between a rock and a hard place when considering the repair-or-replace question. From an operational standpoint, while some manufacturers claim they are self-priming, most centrifugal pump models cannot prime unless they are first pre-primed. Also, they do not have the capability to fully deplete tanks during unloading, cannot strip lines for product recovery and spill reduction, and cannot run in reverse for recovery between loads. Lastly, centrifugal pumps must operate at a speed that requires either a hydraulic-drive system or a gearbox to convert typical PTO speeds to the required pump speed.
- Sliding Vane: This PD-pump technology features a rotor with retractable vanes that protrude and retract as the rotor turns. This setup draws liquid into chambers that are created by the spaces between the vanes, from where it is pushed to the discharge port. The self-adjusting vanes sustain the pump’s volumetric performance, making it energy efficient while simultaneously preventing product slip. Another feature of sliding vane pumps is a lack of metal-to-metal contact, which reduces the possibility that pump friction and galling will occur. An additional sliding vane attribute is its self-priming ability and suction-lift capabilities, which allow the creation of an internal vacuum strong enough to strip lines and tanks. Since there is no metal-on-metal contact inside the pump, sliding vane pumps have a liquid-handling range from ultra-thin liquids (0.2 cP) all the way up to liquids with a thickness of 22,500 cP.
Not all pump technologies are created equal. Knockoff models have entered the market that may look the same as a high-end pump (while costing less), but their performance leaves a lot to be desired. Mainly, they fail quicker during normal operation. Because of their reduced purchase cost, these sliding vane pump models will likely not be run to failure and then replaced, rather than repaired.
Choosing a Pump
There are a few variables that must be considered in picking the best pump type. The first is purchase price. A pump that is designed to be repaired will have a higher purchase price than throwaway technologies. Developers and manufacturers of pumps designed to be repaired take great pains to make the purchase price as palatable as possible, knowing that any excess upfront costs can be recouped on the back end via longer service life and more manageable and bottom-line-friendly repair costs.
However, complicated or reoccurring component repair or replacement also brings the potential for a large amount of ancillary costs for the fleet owner. The more complicated the pump and its drive system, the less likely that fleet operators—which are often lean 3-4 truck operations with small staffs—will have the expertise to perform repairs or preventive maintenance. In this case, an outside service provider will need to be scheduled, or the pump shipped out for repairs, which can mean excessive idle time for the truck, along with significant labor costs.
All of this sounds like an argument to simply replace the failed pump with a new one, but that returns us to the first consideration of purchase cost, acknowledging again that initial cost may be lower, but over the 20-year life cycle of a high-quality sliding vane pump, an external gear or centrifugal pump needs to be replaced three or four times.
In general, many people take a sense of pride in keeping a piece of mechanical equipment operating over an extended period. Sure, that snow blower may be 30 years old, but by taking care of the impeller, it hasn’t failed yet. Or there are instances of fuel-oil suppliers who will re-chassis their transport trucks but install their 10-year-old sliding vane pump on the new chassis.
Chris Hordyk is a product manager for Blackmer. He can be reached at firstname.lastname@example.org. Blackmer is a leading global provider of sliding vane, gear, regenerative turbine and centrifugal pump, and reciprocating compressor technologies for the transfer of liquids and gases. Learn more at blackmer.com and psgdover.com.