To improve the performance of any piece of equipment requires a complete understanding of its operation and the effect on its component parts. The definition of mechanical reliability is the probability that a component, device or system will perform its prescribed service without failure for a given time when operated correctly in a specified environment.
A component part is the smallest part that would normally be replaced. A device, such as a pump, compressor, agitator, mixer, etc., is made up of many component parts. A system, such as a process plant, refinery, power plant, ship, etc. is made up of many devices. Thus, when a critical component fails, it can have a tremendous economic impact, not only on the device in which it is installed, but on an entire system. A mechanical seal is just such a component. The major causes for seal failure on a pump are a result of the following conditions:
Identifying the cause for failure will lead to significant savings for the user.
As the shaft of a pump begins to rotate, a small fluid film develops between the seal faces along with unwanted frictional heat from the seal surfaces in sliding contact. If the amount of frictional heat developed at the seal faces cannot be removed, then the liquid being sealed will flash to a gas or begin to carbonize. Developed frictional heat at the seal faces must be removed.
Each contacting seal has an operating envelope, as illustrated in Fig. 1. The upper limit is determined by wear. More importantly, a seal must operate at a temperature to prevent boiling of the liquid sealed. Operation within the envelope will result in excellent seal life.
Operating near boiling point
Common cryogenic fluids such as argon, nitrogen and oxygen are stored near their atmospheric pressure and pumped near their normal boiling points. These are the most common cryogenic fluids used in industry. The fluids are delivered by over-the-road trucks to industrial users and hospitals. Each truck uses a single stage centrifugal pump driven by a hydraulic motor to move these liquids from the truck to the storage tanks. One fleet operator with 25 trucks began an aggressive program to reduce failures and improve equipment reliability. An analysis of the operation’s seal life and repair costs is shown in Table I. Not only were the maintenance costs excessive, there were also financial losses when deliveries could not be made.
Upon reviewing the seals that failed in this cryogenic service, it became clear that at certain times during the operation of the pump the fluid at the seal faces was flashing and extreme wear and heat checking occurred on the mating ring in the seal assembly. Further complicating the problem was the cool-down period for the equipment. Both the pump and piping had to be cooled down to the liquid gas temperature. Any rise in product temperature could have led to the pump cavitating and the seal running dry.
To be successful in operating near the boiling point of the fluid being sealed requires a seal that eliminates the frictional heat from the sliding services in contact. A seal that is in a controlled environment will allow the liquid to turn to a gas without violent flashing. The properties of the cryogenic fluids to be sealed are given in Table II.
The success from using a non-contacting seal can be explained by reviewing the vapor pressure curve for nitrogen shown in Fig. 2. In this case, nitrogen that is being transported by tank truck is normally at 30 psig/2bar and -320 F/-190 C. When using a contacting seal, the temperature increase at the seal faces is sufficient to start the boiling process at pumping pressure. In an uncontrolled environment such as a contacting seal, continuous flashing damages the seal faces, shortening seal life. During operation of the non-contacting seal design, the temperature rise at the seal faces is only a few degrees, eliminating violent flashing of the cryogenic liquid.
The savings associated with improved seal reliability for the 25 trucks in this cryogenic delivery fleet operation are shown in Table I. These savings were substantial enough to allow the purchase of a new tank truck.
Operating in a poor mechanical environment
A poor mechanical environment requires a seal to move an abnormal amount during operation. The motion transmitted to a seal can be angular or axial. The most common cause of angular motion is piping stresses transferred to the pump casing. This type of loading will result in premature seal failure.
In one case, a power plant experienced a seal failure every three months. Measurements taken on the pump casing at full operating pressure and temperature indicated 0.016” of deflection. This, in turn, distorted the seal chamber and mating face.
The estimated angular distortion or out-of-squareness at the seal face was greater than 0.012”. The shaft was turning at 1800 RPM. This meant that the seal had to flex 0.012” of travel 1800 times/per minute.
The solution to this problem was to add an expansion joint in the piping in the suction line to the pump, which would eliminate the high load being transferred to the pump casing. Clearly, this failure had nothing to do with the design of the component parts of the seal. The savings per year per pump were estimated to be $18,000.
Excessive axial motion can result in the loss of a seal. There are two types of axial motion to consider:
Axial motion from thermal growth of equipment can cause the seal to run solid, resulting in failure. This is more likely to occur on large pieces of equipment. High thrust bearing wear might be expected on a high-speed boiler feed pump, where, over time, it could lead to seal failure.
A ship’s power plant, with low boiler demands, is a prime example of where axial shaft motion might occur. The greater the wear on the thrust bearing, the more axial travel the seal must handle. When the travel is excessive, the seal will run solid and fail. The cost to the ship’s power plant would be excessive.
Improved compressor performance
A synthetic fuel processing plant implemented a program to reduce maintenance costs and improve the reliability of two large compressors vital to plant operation. The gas compressors can reach process temperatures of 650 F and 370 psia respectively. Steam is used as a buffer fluid to prevent gas in the compressor from reaching atmosphere. Steam pressure is 10 psi above the process gas pressure. At these conditions, steam cutting of the existing sealing surfaces was occurring. Annual maintenance to replace the existing seal was $25,000. Annual bearing repair was $12,500. The annual cost of steam was $100,000.
Review of existing non-contact seal technology determined that it could be redesigned to handle high temperatures. Both compressors were converted to the new technology. Each compressor subsequently operated successfully for 10 years without any major work required. The $2,470,000 in savings over this time period reflected a significant payback from implementation of dry-gas sealing technology for high temperature services. The first compressor will be overhauled this year and the second compressor next year.
As shown by these short case study examples, substantial savings can be achieved by analyzing the reasons for short equipment life and applying the best solution. By the same token, improper specification, application and maintenance of critical components like mechanical seals can lead to reduced reliability and substantial losses for an operation. MT
1. Wallace, N.M, Redpath, D., and Netzel, J.P., 2000, “Toward Reduced Pump Operating Costs,” 17th International Pump Users Symposium Texas A&M, Houston, TX
2. Netzel, J.P., Redpath, D., and Wallace, N.M., 2001, “Toward Reduced Pump Operating Costs – Part 2 Avoiding Premature Failures,” 18th International Pump Users Symposium, Texas A&M, Houston, TX
3. Netzel, J.P., and Voigt, J., 2001, “Reducing Life Cycle Costs For Pumps Handling Cryogenic Fluids,” 18th International Pump Users Symposium, Texas A&M, Houston, TX
4. De Maria, R., and Peterson, K., “Improved Compressor Containment With Dry-Gas Seals,” Hydrocarbon Processing, December 1999