Efficient mechanical seals aid reliability

Here Chris Dean, technical training officer for AESSEAL®, looks at how these can be used by engineers and maintenance teams to improve pump reliability, minimise downtime and so cut unnecessary costs.


Pin wear affects the efficiency of both pump and seal.
 

Production efficiency is a key focus in the increasingly competitive manufacturing sector because the financial performance of a company is directly linked to the amount of product that is shipped from the factory gate. Any downtime has a significant effect on the company and, ultimately, shareholder return.

However, often it is these same production pressures that can cause premature pump failure. For instance, focusing on production uptime can lead to rushed repairs or poor re-installation practices, causing unwanted vibration. These simple time pressures – getting the repair done as quickly as possible so that production can be up and running again – occasionally means insufficient time is allocated to simple, but crucial tasks.

As a result, in an increasingly fast paced production environment, one of the key items for achieving optimum pump reliability is to ensure the mechanical seal (the seal between the rotating shaft and the pump casing) is operated in an optimal manner. There are some key fundamentals, such as the fact that the seal faces of mechanical seals need to be kept as clean and well lubricated as possible.

This is achieved by using the pumped product in single seals or a known liquid in dual seals. Seal face lubrication quality is also affected by temperature so keeping seal faces as cool as possible is critical. Put simply, the higher the quality of lubrication between the faces, the longer the mechanical seal will last. This lubrication interface is known as fluid film.

The fluid film, which provides the lubrication between the faces, minimises face contact and reduces heat generation and as such needs to be kept clean to prevent abrasive particles entering the seal faces.

Vibration is another area that can significantly affect the life of the mechanical seal and so pump reliability. Continuous vibration will cause the gap between the seal faces to open sporadically, allowing ingress of particulates between the faces and poor lubrication stability. If particulates get between the seal faces this will cause irreparable damage to the faces. Equally, if the fluid film is unstable, the two seal faces can collide, potentially causing impact damage.

A common question we are asked is how long should a mechanical seal last? The answer to this is that it should last until the faces wear out. That means seal life is directly affected by the quality of lubrication and this in turn by a number of basic factors: temperature, pressure, vibration and speed. Here we look at some of the issues that influence these variables.

Pump reliability often depends upon the environment in which they are used.

 

Vibration

Identification and elimination of vibration is critical to maintaining the health of the seal and equipment. Time spent eliminating vibration will result in lower maintenance costs and increased uptime, whilst ensuring optimum life of the mechanical seal.

Shaft alignment

At least a half of vibration-based damage to rotating machinery is directly related to misalignment of the equipment.
As shown in Figure 2, the shaft on this multiple bearing assembly is misaligned. This introduces load onto the bearings and imparts vibration into the equipment. The resultant vibration transmits not only into the bearings but also the mechanical seal, causing the seal faces to become unstable, which will result in damage.

Cavitation

This is a frequent problem with pumps and occurs where there is inadequate net-positive suction head to the pump and the liquid product changes into a vapor state at the impeller. This reduced pressure inside the pump causes the fluid to boil at low temperatures and the minute bubbles implode at the speed of sound, causing significant levels of vibration.

Along with vibration, other indications that process fluid inside the system is cavitating, include a noise similar to pumping gravel, a loss in capacity, and reduced electrical consumption as less volume of fluid is being pumped. Ultimately, cavitation leads to damage to the seal, impeller, casing and bearings.

Causes of cavitation vary from poor suction pipe work design, low suction tank level and the process fluid being close to boiling point. Alongside this are issues such as build-up in the suction pipe work, poor installation and blocked, or partially closed, suction valves.

Another situation where vibration is present is when pumps share common pipe work. An example is a duty pump running adjacent to the standby pump. As the duty pump runs it causes vibration which is transmitted into the standby pump because of its close proximity. As it does this, it can flatten the bearings in the pump (known as false brinelling), a process indicated by material damage over an extended time, even though the standby pump is not being used.

Aside from the bearings in the pump, the mechanical seal is also being vibrated, and this can transfer into multiple pumps if they share the same mezzanine deck. The vibration now begins to compromise the life expectancy of the pump bearings and as soon as the equipment is started the flat spots on the bearings caused by false brinelling now result in vibration on the mechanical seal.

This results in the seal faces opening, allowing contamination to enter and subsequent damage.

No flow

In applications where the process fluid is used to lubricate the seal faces, lack of, or no flow through the pump results in reduced lubrication to the seal faces and therefore damage. This is often caused by simple factors such as the suction valve not being opened on the piece of equipment. The result is that the faces instantly touch each other, due to a lack of lubrication, causing high heat generation and seal face distortion with subsequent damage to the O-rings.

There are other common reasons why there is no flow through a pump. For example, one of the discharge valves not being opened or the strainer or filter not being cleaned. If there is a prolonged lack of liquid flow through the pump the temperature inside the pump continues to increase, which again exacerbates the damage to the seal faces. If it is not one of these, check that the tank hasn’t been allowed to run dry by being pumped past where it should have stopped because level switches no longer work. Sometimes it can simply be a case of blockage in the pipework.

Solids and abrasion

Standard single seals frequently use a soft carbon face that can be damaged by hard, abrasive particles. In highly abrasive liquids we install two hard faces to resist erosion, but we still don’t want particulates between the seal faces. These particles cause scratches on the face, which result in the seal leaking. The faces are supposed to act as a strainer and filter out particles. The objective, therefore, is to keep these solid, abrasive particles out of the fluid film. But if the equipment is allowed to vibrate, solids from the process will contaminate the fluid film and result in particles ending up between the seal faces.

We can see there are several ways to compromise the performance of a mechanical seal. Principally, these are poor shaft alignment, vibration, no flow of process fluid and presence of abrasive particles. Seals are precision engineered components with a lubricating fluid film thickness of around 1 micron (0.001mm) As such they are not designed to tolerate operating in the situations described above.

Frequent seal failure is an indication that something more serious is wrong in the system. As such it makes sense to think of a mechanical seal like a fuse in an electrical system. When a fuse blows it can be because of a whole variety of reasons such as water on connections or a power surge – not because there is a fault with the fuse.

It is a healthy approach to view a mechanical seal in the same way. Failure of a mechanical seal is simply an indication that there is an issue somewhere else in the system. Having a planned maintenance programme in place that identifies these factors before they create an issue is the best way of ensuring the performance of the mechanical seal, which leads to improved pump reliability and minimises unplanned downtime.