A high start frequency accelerates pump wear considerably. Every time a pump starts, it goes through a short period of dry or poorly lubricated running, high mechanical load and thermal fluctuation. This mainly affects the mechanical seal, but the bearings, motor and shaft suffer too. In this article we answer the most common questions about start frequency and pump wear.
How many starts per hour can a pump handle?
Most industrial pumps are designed for a maximum of 10 to 20 starts per hour, depending on the pump type, motor size and manufacturer’s specifications. If you regularly exceed that limit, wear on both the motor and the internal components increases quickly. For smaller pumps with compact motors, the limit is often even lower.
The exact limit is usually given in the manufacturer’s technical documentation. Bear in mind that this is a theoretical maximum under ideal conditions. In practice, with changing fluid temperatures, variable pressures and ageing components, it is wise to stay well below that limit. In many industrial applications, more than 10 starts per hour is already a signal to reassess the system.
Why does a high start frequency accelerate mechanical seal wear?
At every pump start there is a short period in which the mechanical seal is not yet fully lubricated by the process fluid. In that fraction of a second, the seal faces run dry or semi-dry against each other. This causes microscopic wear on the sealing surface. The more often this happens, the faster the service life of the mechanical seal declines.
Each start also delivers a sudden mechanical shock to the seal. The shaft accelerates from standstill to operating speed, placing a short but intense load on the seal. With a high start frequency, these shocks add up. The seal faces, often made of materials such as silicon carbide or carbon, are hard but sensitive to repeated thermal and mechanical cycling.
An additional effect is heat build-up. Every start generates frictional heat. If the pump restarts before the seal has fully cooled down, the next start cycle begins at an already elevated base temperature. This speeds up the degradation of the sealing materials and increases the risk of leakage.
Which other pump components suffer from too many starts?
Besides the mechanical seal, the bearings, motor and shaft are the components that suffer most from an excessive start frequency. Each of these parts experiences a peak load at every start that does not occur during normal operation.
- Bearings: At start-up, the bearing carries the full radial and axial forces before the fluid film has fully built up. Repeated starts wear the bearing material faster than continuous operation.
- Motor: Every start draws an inrush current several times higher than the rated operating current. This heats up the motor windings. Too many starts per hour leave the motor too little time to cool down, which damages the insulation and shortens its service life.
- Shaft: The sudden torsional load at every start puts mechanical stress on the shaft. With a high start frequency, this can eventually lead to fatigue cracks, especially if the shaft already has minor damage.
- Coupling: Flexible couplings absorb the shock at start-up, but as a result they also wear faster with a high start frequency.
How do you recognise wear caused by an excessive start frequency?
You can recognise wear caused by an excessive start frequency by a combination of early mechanical seal leakage, higher bearing temperatures, unusual vibration and shorter intervals between maintenance than the manufacturer specifies. These signs indicate that the pump is consistently under a heavier load than it was designed for.
Specific indicators to look out for:
- Mechanical seal leakage earlier than expected based on running hours
- Visible wear or discolouration of the seal faces when inspecting the seal
- Higher motor temperature or frequent tripping of the thermal protection
- Increased vibration levels measured at the bearing housings
- Bearings that need replacing sooner than planned
If you recognise these patterns, it is worth logging the start frequency and comparing it with the manufacturer’s specifications. This often shows that the pump starts too often on a regular basis, which is the direct cause of the accelerated wear.
What are the best ways to reduce unnecessary pump starts?
The most effective ways to reduce unnecessary pump starts are installing a variable frequency drive (VFD), enlarging the buffer tank or adjusting the control so the pump runs longer at a lower speed instead of constantly starting and stopping.
Practical measures at a glance:
- Install a variable frequency drive: A VFD adjusts the pump speed to demand. The pump no longer needs to switch on and off, but runs continuously at a variable speed. This considerably extends the service life of all components.
- Enlarge the buffer tank: A larger buffer or expansion vessel means the pump needs to start less often to maintain pressure.
- Adjust the control strategy: Set the cut-in and cut-out pressures further apart so the pump runs longer per cycle.
- Optimise the process: Check whether demand on the pump can be made more even through process changes upstream.
- Set a minimum off time: Make sure the pump has a minimum waiting time after a stop before it is allowed to restart, so the seal and motor can cool down.
When is refurbishment or replacement of the mechanical seal necessary?
Refurbishment or replacement of the mechanical seal is necessary as soon as you notice leakage, the seal faces are visibly damaged, or the seal has reached the end of its expected service life due to a high start frequency. Do not wait until leakage leads to pump downtime or damage to other components.
Specific situations in which you should take action straight away:
- Visible leakage along the seal, even if it still seems minor
- Seal faces showing scratches, pitting or thermal damage on inspection
- The seal has run fewer hours than the minimum service life specified by the manufacturer
- An increasing tendency to leak after a period of high start frequency
If in doubt, a visual inspection of the seal faces is the quickest way to judge whether refurbishment makes sense. In many cases, refurbished seals perform as well as new ones, especially when the seal faces are re-lapped and the secondary seals are replaced.
How we help with wear caused by a high start frequency
At Qseals, we regularly see mechanical seals come in that have worn prematurely because of an excessive start frequency. We are happy to help you find the cause and choose the right solution, whether that is a refurbishment, a modified seal or a different choice of material.
What we can do for you:
- Inspection and diagnosis of the seal to determine whether start frequency is causing the wear
- Refurbishment of virtually all brands and types of mechanical seals, including lapping of seal faces
- Advice on material selection for seal faces that better withstand repeated dry starts
- Engineering of custom solutions if the standard seal is not suitable for your application
- Delivery of new seals from our stock of more than 200,000 products for fast replacement
Dealing with a pump that starts too often or a mechanical seal that wears faster than expected? Contact us for honest advice and a practical solution.
