Failing Mechanical Seals – Implications On Performance

Failing of mechanical seals can have major implications on the performance of pumps. Here we look at a client whose power station has 12 booster pumps that feed their Sulzer HPT Pok30 boiler feed pumps where they have suffered numerous seal failures.

Project Description

The implication of these mechanical seals failing would mean that the unit would be required to be taken offline for seal replacement. With the possibility that if one of the other units has already been taken offline for other maintenance reasons it would mean that the boiler would need to be shut down, this would have a large impact on the overall power station output.

These booster pumps were originally designed for gland packing and then upgraded to mechanical seals at installation.

There are a number of the John Crane double seals installed in the existing booster pumps and some of these units have been installed for a considerable period of time. If the seals were plumbed up correctly in all instances, seal longevity would have been even greater.

Single mechanical seals consist of two dynamic surfaces pressed together by a spring; once these faces degrade the seal will leak and ultimately fail.

The idea of the double seal is that you have two independent seals so that when the inboard seal fails, you have a back up seal.

Double Mechanical Seal

The inboard seal fails because it is the seal that sees the feed water pressure and temperature and essentially does all the work. The outboard seal just idles along as this is only seeing low pressure and ambient temperature cooling water. The idea is that when the inboard seal fails, the outboard seal holds the boiler feed water and contains it until a station maintenance plan can be put together to change the seal out.

The use of a double mechanical seals allows the operators to monitor the seal water, looking for signs of discoloration to give an indication that the primary seal has started to leak. This boiler feed water would have to be disposed of, but, unlike the single mechanical seals, the collected boiler feed water could be disposed of in an environmentally friendly manner.

It must be noted, a double mechanical seal requires auxiliary water for lubricating this may fail if water is not available. To prevent such conditions, a small inline filter could be engaged. There are other systems that will provide a visual or audible confirmation that your inboard seal has failed.

The only current way the power station can tell that the inboard seal has failed is because the cooling water out pipe gets increasing hot, because the boiler feed water is actually coming from the outlet pipe, hence the defect notes for hot pipes.

Analyses and Results

We found in the way these booster pumps are set up with the cooling water feed lines, there appears to be a couple of pumps with both inlet and outlet lines that are hot. If this is the case then it means that the inboard seal has failed and that the stuffing box pressure would be higher than the seal water inlet and hot process water is going back into the lines.

The double seals that are being used, really need to be upgraded because at present it appears that the set up doesn’t allow personnel to tell when the inboard seal has failed and all the advantages of a double seal are not being utilised such as:

Safety because of the secondary outboard seal, if single seals are used and the seal fails, you have 160-degree hot water spraying out of the pump.
With double seals, if the double seal inboard seal fails, then the outboard secondary seal will contain the hot process water and allow the power station to put a maintenance plan together to change the seal out and create planned maintenance over crisis maintenance.
Reliability increases for the same reason as above.

Project Conclusion

There should be non-return valves installed on all feed lines to the seal. If the stuffing box pressure is higher than the cooling water pressure, the process hot water will continue to feed into the cooling water supply when the inboard seal fails, regardless of whether the power station stay with the existing 5620 double seal or upgrade to the 5620P seal with a pumping ring.

We in conjunction with John Crane Australia have suggested that the power station change the seal arrangement to the 5620P seal, which is essentially the same as the double seals currently being used on a couple of their booster pumps. It has the benefit of an installed internal pumping ring in the seal.

Additionally a Safe unit should be installed in the feed line on the cooling water inlet so that the flow rate to the seal can be monitored and adjusted along with a Smart-flow unit. The Smart-flow unit will dramatically reduce the water usage by some 98% and will provide the power station with the ability to be able to tell when the inboard seal has failed.

The Smart-flow units are available with an audible or visual alarm, depending on which option you chose. The Smart-flow works by having the cooling water inlet plumbed to the Smart-flow. This means there is no water just pouring from the seal outlet going to waste as it is contained by the Smart-flow.

When the cooling water that is in between the inboard seal and outboard seal temperature rises to between 60-80 degrees C, a thermo valve opens and releases around 190ml water, the reason it does this is water loses the lubricating properties at around 90 degrees C. The Smart-flow allows a refill of fresh cooling water into the seal and does this 24/7 automatically. The pressure kit is a once only purchase.

Smart-flow Pressure Kit with the option of an alarm is our best recommendation, as we believe the combination of the two items together will provide the power station with a superior system that will provide a visual or audible confirmation that the inboard seal has failed.