The process industries rely on numerous pumps to perform reliably to maintain plant productivity. To do this, these pumps must be properly sealed to prevent leaks and contamination of process fluids. They also must be capable of handling system upsets, operating under often harsh conditions, be easy to install and provide a low cost of ownership over their useful lives.
The two most common means of sealing pumps are mechanical seals and compression packing. Unfortunately, neither of these popular solutions is capable of meeting all these requirements. Mechanical seals deliver adequate performance, but are subject to leaks, hidden costs and instantaneous failure without prior indication. Moreover when they fail, they cannot be repaired. Because of this vulnerability to sudden, unexpected failure, many plants stock spare pumps to minimize disruption of their operations.
Mechanical seals are adversely impacted by a number of factors, including shaft run-out, vibration, abrasive and viscous media, cavitation and air entrapment. Left uncorrected, any of these factors can cause mechanical seals to fail. Excessive shaft run-out requires realignment or bearing replacement, both of which are costly. Expansion joints can be installed to dampen vibration, but they too are expensive and labour-intensive.
Double mechanical seals are often used for pumps in abrasive service, but ingress of process media can cause them to fail. They should not be used in applications involving viscous media, nor are they recommended for service in pumps subject to internal cavitation. Excluding air and eliminating entrapped air bubbles from the process fluid requires rerouting both the media and flush water, and purging the pump’s stuffing box.
Mechanical seals that require flushing cannot withstand dry conditions or contamination by particulate matter that can damage their faces. Most mechanical seals are one-piece, so installing them requires the pump to be removed from service and taken apart. In addition, installation of these complex seals often requires on-site technical support from the manufacturer.
Prior to the advent of mechanical seals, the most popular method of sealing pumps was compression packing. It is still used, but like mechanical seals it has a number of disadvantages, most notably excessive water consumption. Interestingly, some users of compression packing seem more concerned with diverting leaks than stopping them.
Certain types of packing sets must be flushed to keep them from overheating and the packing-shaft interface lubricated. Water consumption using these sets can be as much as seven gallons a minute and the flush water must be removed from the process fluid and treated. This removal and treatment requires costly equipment and results in lost productivity. If process fluid instead of flush water is used for lubrication, any loss of the fluid usually cannot be recovered.
The sealing dynamics of packing sets involve compression and radial expansion, which creates friction between the packing and pump shaft. It can take up to twice the power consumption of a pump with a balanced mechanical seal to overcome the resulting drag. In addition, excessive radial expansion can cause wear on the shaft, requiring it to be replaced or a new sleeve to be installed.
Notwithstanding these limitations, compression packing sets are often preferable to mechanical seals. They rarely fail without exhibiting prior warning signs. They can be adjusted to control leakage and withstand shaft run-out, vibration, abrasives, cavitation, and loss of flush water better than mechanical seals. In addition, they are easy to install, and can be repacked with standard spooled packing, unlike mechanical seals that have to be supplied to exact pump dimensions.
A rotary shaft sealing system that outperforms both mechanical seals and compression packing sets in flushed abrasive services does not require water to lubricate seal surfaces. Instead it forms a bearing relationship with the face of the shaft in the seal chamber, excluding process media from the stuffing box. This radial bearing-to-face design is both less expensive and more effective than traditional mechanical seals, providing cool, dry operation with minimal dilution of the media. Requiring half the power consumption of packing sets, these rotary seals are as energy-efficient as balanced mechanical seals, and use 60 times less flush water than comparable compression packing.
In addition they can be installed in an hour compared with two to three for mechanical seals. Installation requires no special equipment or start-up adjustments, and the seals can be adjusted to control leaks while a pump is running.
One chemical plant used these seals to replace mechanical seals in the ANSI pumps in its brine system. At £1,000 each, the titanium double mechanical seals it had been using lasted a mere six weeks in the corrosive media, costing the plant more than £70,000 a year on just seals. At a fraction of the cost, the rotary seals last up to three times as long and cost less to install than the mechanical seals they replaced, yielding annual savings of more than £70,000. The conversion also increased equipment uptime and made the plant safer.
Rotary seals will never displace mechanical seals and conventional compression packing in all process applications, but they are a better option in those that involve abrasion and corrosive environments.
Under these conditions, they provide more reliable, cost-effective performance with far less maintenance, water consumption, and risk of catastrophic failure.