When dry running leads to catastrophic failure, all options need to be reviewed

In 2015 Kelair began working with a major chemical supplier on a project to improve the reliability and performance of truck mounted 50% Sodium Hydroxide (Caustic) unloading pumps.

They had been using cast iron DIN standard centrifugal close coupled and long coupled pumps fitted with single hard faced mechanical seals which were failing with great frequency and typically the repairs would be close to the cost of replacing the pump with no improvement in reliability.

Truck mounted chemical pumps face a variety of challenges; they can operate against varied heads, depending on the site they are delivering too. Dry running is a very real possibility and it is difficult to apply dry run protection to pumps in this application as the increase in complexity creates problems of its own.

Our first contact was quoting to repair one of the pumps and rather than supply more of the same, the client invited us to sit down with their operators and engineers to work toward a long term solution.

The First Step

The first step was to do some desk top work on the existing pumps, discovering that reducing the impeller diameter of the pumps would allow them to operate on a stable section of the curve and ensure the pumps would operate without cavitation.

While the single hard faced seal (sic/sic/metal bellows) is a good choice for caustic service, Silicon carbide vs Silicon carbide heats up very quickly when run dry which results in catastrophic failure in a very short period of time. We resolved that the seal was not suitable and we were to seek a more suitable alternative. It should be noted that it was impractical to change the pump type at this stage because of existing pipework and truck build.

First Option May Not Be Best Option

Our first option was to look at a double mechanical seal, it would be perfect for the fluid and would stand dry running. However, double mechanical seals require a support system which meant mounting a buffer liquid vessel and connecting pipework. The complexity of installation and ongoing maintenance required was unappealing so we looked for another option which could tolerate the inevitable short periods of dry running while retaining simplicity in operation.

We resolved to install a Universal Cartridge Wet/Dry Running seal. The Wet/Dry seal uses Carbon vs Silicon Carbide faces. The carbon primary face enables up to 6mm of wear which would sacrifice a small amount of material when the pump ran dry rather than offering Silicon carbide vs Silicon carbide faces which virtually never wear but destroy themselves when run dry. The pump was re-installed and its performance reviewed.

New Options

In 2016, the client asked that we revisit the caustic loading service, but this time for new trailers so there would be no pipework or build constraints. We were instructed to investigate multiple pump types and provide pro’s and con’s for each.

We reviewed the following:

  • Helical Rotor Pump - The performance and cost were favourable and the seal was not susceptible because of the lower speed. However, physical size was an issue and the pump would require electronic dry run protection.

  • Internal Gear Pump - A high speed gear pump was suitable for the duty. However, it would not accept a cartridge mechanical seal and the pump itself would have similar dry running issues to the helical rotor pump.
  • Seal/Less Magnetic Drive Pump - Magnetic drive pumps are ideal for caustic. Unfortunately dry running usually results in catastrophic failure, which is why we always discuss Digital Load Monitors with magnetic drive applications.
  • End Suction Centrifugal Mechanical Seal - The concept of a centrifugal pump fitted with a Wet/Dry Running mechanical seal had already proved itself in service but in this case we were not constrained to the DIN standard pump and chose a Heavy Duty ISO 5199 standard end suction centrifugal which was more fitting for a chemical service.

The End Result

The pump was provided in cast iron construction with a 316 stainless steel impeller fitted with Wet/Dry Running cartridge mechanical seal and API Plan 11 flush mounted onto a fabricated steel base with an IP66 electric motor.

The end result is the proper application of centrifugal pumps as a well understood technology fitted with a reliable single seal presented as a simple solution to a very difficult problem.

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