Pump Start Up & Seal Failure

I’ve started my pump, and now it’s leaking . . .
Or even worse “I’ve started my pumps, and they are all leaking.” The next question is “when can this be fixed?” but the real question should be “why did the seal fail?” There are a variety conditions that can lead to seal failure, and the majority are preventable.

First, lets examine the conditions that are not preventable in the field. This could include a bad seal. It does happen, just not very often. Mean failure rates of the mechanical seals used in Bell & Gossett pumps are extremely low. Another possibility is an improperly constructed seal chamber in the pump itself. Out of thousands of pumps sold over the last few years, I am aware of one occasion wherein this was the case. While material failure is always a possibility, this is an extremely rare occurrence in our company’s experience.

The article will address typical start-up conditions and cleaning procedures for hydronic systems, basic water treatment of these systems, and the effect that the procedures and treatment will have on the pumps mechanical seals.

Start-up conditions, procedures and water treatment
Changes in new construction, particularly with regards to scheduling, have virtually guaranteed a pump seal failure soon after start-up. The rapid pace of new construction does not allow for use of flush pumps. Flush pumps require a pipe side stream with the appropriate valving to operate, and this does take time to construct and then remove. Please take a moment to consider when you last saw a flush pump being used on new construction, retrofit or T.I. work? When did you last see the use of a flush pump specified? In some informal surveying, five years was the most popular answer, and many people answered ten years.

This means that the system pumps have been used for conditions other than what they were designed for, namely system flush and cleaning. These conditions, and the accompanying fluid characteristics, present significant problems for pump seals.

To begin, one of the first things that will occur in a new system is a pressure test. Water is introduced into the system piping through the pump. This water is dirty, with high levels of sediment and construction debris, and is passed over the pump seal. This fluid contains high levels of dissolved and suspended solids. Mechanical seals are not tolerant of dissolved solid levels above 1000 ppm or suspended solids above 20 ppm, but we will address that in greater detail later.

System cleaning and pipe treatment are typically done with alkaline cleaners. Alkaline cleaning uses chemicals to raise the pH level in the system to somewhere between 9.5 and 10.5, with this level being most beneficial to the pipe.
A quick review of pH –

By definition pH is the measure of free hydrogen activity in water, and can be expressed as : pH = -log[H+]. A more practical statement for our purposes is that pH is the measure of acidity or alkalinity. Measured on a scale of 0-14, solutions with a pH of less than 7.0 are acids while solutions with a pH of greater than 7.0 are bases (alkaline). This is a logarithmic scale, and as such each unit represents a 10-fold increase or decrease relative to the next unit. For example, seawater with a pH of 8 is 10 times more alkaline than fresh, clean water, with a pH of 7. This is noted to illustrate how a pH increase or decrease of 1 or 1.5 is very significant with regards to the fluid properties.

Corrosion levels increase in copper and steel as pH falls below 7. This is acidic, and should make sense on an intuitive level. Since we don’t fill our systems with acids, this is a non-factor. Corrosion occurs readily at pH levels between 7 and 8.9, but falls off dramatically at 9, so the alkaline side of water treatment is where our attention should be focused.

The pH target is 9, with significant corrosion consequences if the system pH should fall below this number. As such, the most common treatment pH target range is between 9.5 and 10.5. Spikes in pH can occur, but for the most part this is not harmful to the piping. It can however be extremely harmful to the pump seal.

Mechanical Seal Tolerances
Internally flushed mechanical seals include a stationary face and a rotating element. The materials from which the seal are made have specific tolerance ranges with regards to temperature, pH and particulate levels. A standard seal, which will have carbon stationary element and a ceramic rotating element, has the following limitations:

· Maximum temperature: 225ºF
· pH range: 7 – 9
· Total dissolved solids: 1000 ppm
· Total suspended solids: 20 ppm

Based on what has already been outlined regarding start-up and water treatment, it can be reasonably assumed that the seals have already been subjected to conditions that exceed there limitations, and probably for an extended period. The water chemistry during cleaning will quite likely have a negative effect on the seal a few months later.

At elevated pH levels, the binder in the ceramic material is leached out, leaving the surface slightly softened and porous. As this element rotates, pieces of the ceramic embed in the carbon face and act like a grinding wheel on the weakened mating face. This will lead to seal leakage. Once the pH spike occurs, the damage is done regardless of subsequent adjustments.
The flushing water that cools and lubricates the seal evaporates due to the frictional heat generated between the seal faces, leaving behind whatever chemicals are dissolved in the water. For this reason, chemical concentrations of dissolved solids above 1000 ppm will act as an abrasive on the seal faces. This will erode the face, and the seal will leak. Similarly, with suspended solids, they can work between the faces and be trapped by the evaporation of the system fluid. The seal will be damaged as consequence.

If high pH levels and elevated concentrations of solids are typical of the installation, other seal materials are available as an option. Like anything, this will come at a price. These seals cost more, are more susceptible to thermal shock, and will crack quite readily if run dry even for a moment. The most common alternative seal would be of EPR/tungsten carbide construction. EPR, ethylene propylene rubber elastomer, is the stationary element. Tungsten carbide is the rotating element. This seal will tolerate pH levels of up to 11 and temperatures to 250ºF. Since the materials are harder, they will handle solids more effectively, but the exact amount is not published.

While we have not discussed temperature in depth, this is not typically an issue on standard hydronic heating systems, but may be a factor in some cogeneration facilities. Please bear in mind that at elevated temperatures, the effects of high pH levels become more pronounced, as well as the evaporation rate of the fluid between the seal faces. These seals will fail more rapidly as a result.

Conclusions and Solutions
How to cure the problem? The most cost effective solution is to simply change the seal after substantial completion of the project. While there are options regarding water treatment that will not elevate pH levels, they come at a steep price increase. Given our competitive market place, this is unlikely to be a regular alternative. Monitoring the water chemistry during start-up and system cleaning will provide an early indicator of seal replacement necessity. Please bear in mind that because the system is corrected does not mean that damage has not been done.

This is a topic about which very lengthy articles have been written, and it is impossible to discuss all the factors of seal failure at any length in this newsletter. I would welcome you comment and input on this topic.

by Mike Caffrey
Branch Manager - San Diego
Dawson Co.

References
· B&G Bulletin 4976 “Mechanical Seal Selection Guidelines”
· The Burgmann Dictionary at http://www.burgmann.com/
· The Chesterton web site at http://www.chesterton.com/
· John Crane Company, article “Avoiding Premature Seal Failure”

Thanks to Roy Ahlgren, Director of Training and Education, Bell & Gossett.
Thanks to Mark Pondel, Manager of Field Service, Bell & Gossett.
With Special Thanks to Michael Burns, General Manager, Aquatec.