The reliable sump pump: An achievable ideal?
Published: 27 June, 2007
The Environment Agency's Pollution Prevention Guidelines provide clear instructions for dealing with trade effluent. However, complying with these regulations can be hindered by the notorious unreliability of the pumps used in the waste disposal process. PWE investigates the issue, and the types of pump available for the process.
The sump provides an unglamorous yet essential function in any industrial process or facility. The removal of wastewater and process liquids maintains a clean and safe working environment as well as allowing plants to comply with the EA"s Pollution Prevention Guidelines. Businesses have a duty to ensure that the waste they produce is handled safely and lawfully, and must prevent the escape of waste by storing and transporting it in a controlled and secure manner, so that it cannot enter surface water drains. Failure to do so is an offence, whether accidental or otherwise.
The challenges of sump-emptying
However, the capacity of an industrial business to comply with these regulations can be limited by the reliability of its equipment. The sump pump that transfers site effluent from the sump where it collects, to the foul sewer where it is transported to a sewage treatment works for decontamination, is notoriously unreliable and a breakdown can cause untold damage through extensive flooding as well as lost revenue through downtime.
Unfortunately, such problems can occur far too frequently due to the challenging nature of the effluent: With the sump situated at the lowest point of a process to allow waste to flow in freely, it is also the site where other items will inevitably end their journey. The liquid in the sump therefore often contains grit, oil, rags and various items of debris. For many pump types, such abrasive effluent causes severe wear on the pump, while debris may block it altogether and has the potential to cause the sump to overflow. The resultant flooding can cause polluted waste liquids to enter surface water or groundwater,
Many site managers have overcome this issue by installing secondary pumps that can take over operation should the primary pump become disabled. The secondary pump can also run in unison at time of peak demand. However, as these pumps are usually the same type, they are equally vulnerable to blocking and therefore can’t provide a long-term solution to the issue.
Stirling Leather, a traditional manufacturer of high quality slink lambskin products, experienced this problem on a regular basis. The company used a submersible pump to deal with the acidic effluent produced during their tanning process. This effluent contains a number of chemicals as well as wool residues. Although this residue is not present in large volumes, it caused serious problems by building up on the impeller of the submersible pump and eventually clogging it. An employee was required to leave his duties and spend time unblocking the pump every few hours - adding up to a considerable cost over time.
A peristaltic solution
The submersible is just one type of pump that can be used for this application – others include diaphragm pumps and progressive cavity pumps. These pumps are often the first choice simply because operators know how to handle them, but they are less than satisfactory for sump applications due to the demands outlined above. In fact, they are particularly ill-suited to the task – the dust and grit causes wear in all three pump types, while debris can build up on the impeller of the submersible (as Stirling Leather experienced) and on the valves of the diaphragm pump, obstructing the flow and causing the pumps to block.
An increasingly popular solution to the problem is the peristaltic pump, due to its reliability and capacity to cope with debris without blocking. "Peristaltic technology is designed to handle difficult applications where its non-contacting pump mechanism wins over other pump types”, says Ashley Shepherd, UK sales manager at Watson-Marlow Bredel. “Where conventional pumps use valves to push liquid through them, peristaltic technology employs rollers to compress and release a hose. This means that there is an absolutely clear path of flow with no valves, seals or glands – and therefore nowhere for debris such as rags and stones to get stuck.”
Stirling Leather installed the Watson-Marlow Bredel SPX50 hose pump six years ago, and since then has not had any problems with its effluent disposal whatsoever. Unlike the submersible pump previously used, peristaltic pumps self-prime and can be mounted at ground level. This makes them comparatively much easier to access for maintenance than the submersible, which has to be kept under the fluid level as it won’t self-prime.
Terry Koerner, production director at Stirling Leather comments: “The SPX’s ability to achieve and maintain the specified flow rate is crucial to efficient flocculation. This and the fact that it has no difficulty handling the wool residues makes it the ideal pump for the duty.” He continues: “Though not as cheap as your everyday submersible, it does, at the end of the day, provide us with a much more cost-effective solution.”
An occasional blockage caused by a particularly large solid can usually be cleared simply by reversing the pump, as flow can be redirected simply by changing the rotation of the rollers, and the sludge often found at the bottom of a sump is no problem to pump straight through. “We’ve never had any problems with stoppages caused through blockages whatsoever - it just keeps on running”, says Mr Koerner. “Maintenance, moreover, is limited to replacing the hose, from which we get a very reasonable lifetime of over twelve months. The hose is the only part of the pump to get worn and is relatively cheap to replace.” Chemically resistant hoses can be changed quickly and easily while the pump itself is not affected by the substance, ensuring long-term reliability and endurance.
Shepherd warns: “With sumps performing such an important function in any process, plants simply cannot afford for them to fail.” He concludes: “So all our customers have been incredibly pleased and relieved to have finally found a reliable solution that enables them to comply fully with the requirements of legislation.”
Pump Type | Handles solids without blocking? | Handles abrasives without wear? | Dry running without damage? | Self priming? |
Diaphragm | No | No | No | Yes |
Progressive Cavity | No | No | No | Yes |
Submersible | No | No | No | No |
Peristaltic | Yes | Yes | Yes | Yes |
For more information: www.watson-marlow.co.uk
