Mechanical Seal FAQs
Here are some of the questions we are most frequently asked…and our answers. Challenge us. If you have a different question or you want more specific information about a particular mechanical seal application, please complete the form below or feel free to call us on +44 (0)1204 388 030.
If there are no other factors involved, then mechanical seal life is limited by how long it takes for the carbon face to fully wear. However, mechanical seals rarely wear out because of old age and usually fail due to some other reason well before the carbon wears excessively.
There are many unknown factors that influence the actual service life of a mechanical seal, making it almost impossible to predict. Typically, it is only possible to compare empirical values from similar applications. For example, a seal operated correctly on a clean, stable media such as a light hydrocarbon may have a service life exceeding five years, whereas a seal operating on a highly viscous or abrasive product may last for just a few months.
All mechanical seals leak; they have to. The leakage rate needs to be sufficient to ensure that there is a lubricating fluid film between the lapped seal faces to minimise friction, heat generation and wear, whilst at the same time being low enough to be acceptable.
There are a number of factors that determine an acceptable level of leakage:
- What are the hazards and risks of leakage?
- Is there legislation that specifies limits on emission levels?
- What is the operating environment and provision for cleanliness control?
- Is there significant cost associated with lost product or barrier fluid?
- The actual leakage rate depends on the mechanical seal type and diameter, the properties of fluid being sealed, the shaft speed, the sealed pressure, the operating temperature.
With water, volatile solvents and light hydrocarbons, the leakage is often invisible because it is emitted as vapour. Whereas, fluids that contain dissolved solids, slurries, polymers and heavier hydrocarbons will present visible leakage.
In any mechanical seal application provision must be made for leakage.
Where will the leaked fluid go? Does the mechanical seal have drain ports? Will it be piped to a safe drain or flare? Will it need cleaning away periodically? How will barrier fluid be replenished?
Again, this depends on the mechanical seal type and the application, but generally a mechanical seal has failed if it is leaking excessively and/or cannot maintain the specified pressure.
A mechanical seal can be considered to have failed if:
- There is an excessive loss of sealed product.
- The product or barrier fluid pressure cannot be maintained at the required level.
- There is an excessive loss of barrier fluid.
- There is an excessive flow of barrier gas.
- There is excessive process contamination (barrier fluid, wear debris or other).
As a rule of thumb, “a mechanical seal has failed if it is leaking at a rate of 250 x its theoretical average”
Mechanical seal failures typically fall into three main categories:
- Incorrect operation and process upsets or changes.
- Incorrect fitting and alignment.
- Incorrect (or not quite correct) seal or system selection
The majority of mechanical seal failures fall into the first category with more than 60% of failures being caused by some kind of process upset or a change in operating conditions.
- Mechanical damage – e.g. seal face damage due to excessive loads caused by high pressure and damage to faces and other parts caused by incorrect fitting.
- Corrosion and chemical attack - e.g. crevice corrosion or pitting of metal parts due to exposure to excessive levels of chloride, H2S, etc. or cracked or swollen elastomeric o-rings due chemical incompatibility.
- Wear - e.g. accelerated face wear due to abrasives or wear of driving parts such as pins and keyways due to excessive misalignment.
- Thermal overload – e.g. seal rings may develop thermal cracks or blisters due to excessive heat at the seal faces due to poor heat dissipation. Elastomeric o-rings can become hard, brittle and cracked due to excessive heat.
A single mechanical seal is the simplest, preferred solution for many applications. However, if the sealed product is not suitable for lubricating a single seal or if reliability and/or safety need to be improved, then a multiple seal arrangement is required.
A single mechanical seal has one set of seal faces that are pressurised and lubricated by the sealed product. The seal leaks the sealed product to atmosphere.
A tandem mechanical seal incorporates two mechanical seals, arranged in succession, with an unpressurised buffer fluid circulating between them. The product side seal (also called inboard or primary seal) is pressurised and lubricated by the sealed product like a single seal. The atmosphere side seal (also called outboard or secondary seal) is lubricated by the clean buffer fluid. If the primary seal fails then the secondary seal prevents product leakage to atmosphere, effectively adding an additional level of safety and reliability to a single seal.
A double mechanical seal incorporates two mechanicals seals with a pressurised barrier fluid circulating between them. The barrier fluid pressure is always at a higher pressure than the sealed product. Both the inboard and outboard seal faces are pressurised and lubricated by the clean barrier fluid. If the inboard seal fails then the pressurised barrier fluid leaks into the product. If the outboard seal fails then the pressurised barrier fluid leaks to atmosphere. In either case, the sealed product is prevented from leaking to atmosphere. Double seals are used when a high level of safety is required and where the sealed product is too arduous (abrasive, volatile, viscous, hazardous, etc.) to lubricate mechanical seal faces effectively.
A dual mechanical seal is arranged like a tandem seal with the sealed product around the outside of the inboard seal. However, a dual seal has a pressurised barrier fluid circulating between the two seals. Both the inboard and outboard seals are pressurised and lubricated by a clean, stable barrier fluid.
All mechanical seals are hydraulically balanced to control the opening and closing forces on the seals rings. The measure of hydraulic balance is called balance ratio. A mechanical seal with a balance ratio of one or more is an unbalanced seal, whereas a seal with a balance ratio of less than one is a balanced seal.
The degree of hydraulic balance is determined by controlling the dimensions of the radially disposed areas of the seal rings. The balance ratio is the area of the seal face compared to the net hydraulic closing area.
An unbalanced seal typically has a balance ratio of around 1.2. In this case, the pressure on the seal face is 1.2 x the sealed pressure, plus the pressure created by the spring force. Therefore, the face pressure on an unbalanced seal is always greater than the pressure being sealed. For this reason, unbalanced seals are limited to a pressure of 10 barg and are used on relatively simple, low pressure applications.
A balanced seal typically has a balance ratio from 0.65 to 0.85, so that the seal face pressure is always less than the sealed pressure. This enables the seal to generate sufficient fluid film between the seal faces and therefore allows operation at high pressure.
Generally, liquid lubricated seals should not be dry run regularly or for prolonged periods. However, some mechanical seals with a carbon face will tolerate marginal lubrication and short, intermittent periods of dry running.
If the temperature at the mechanical seal faces becomes excessive due to insufficient heat dissipation then the liquid film between the faces can start to vapourise and become unstable. In this case the seal face materials will contact excessively and begin to wear rapidly under dry running conditions. Many wet seals that use general purpose carbon will not tolerate this and will ultimately fail. Some mechanical seal designs use grades of face material that are more tolerant to intermittent or short periods of dry running during upset conditions.
Some mechanical seal types are designed to run dry continuously and use seal face materials specifically for this purpose, e.g. some top entry mixers incorporate single mechanical seals where the seal faces affectively seal a nitrogen gas blanket and the seal faces are wetted only by vapours.
Applications in the chemical and pharmaceutical industry require a double mechanical seal that operates on a pressurised dry nitrogen barrier gas. For carbon graphite to dry lubricate effectively then moisture must be present, otherwise it will wear rapidly. However, there are grades of carbon graphite available that are specifically formulated to operate on dry nitrogen with minimum face wear.
In most cases carbon graphite vs silicon carbide is the preferred seal face material combination. However, in some applications, carbon graphite is not suitable as a face material, so a combination of hard materials such as silicon carbide or tungsten carbide is required instead. This is usually determined by the characteristics of the sealed product or to meet the requirements of stringent contamination standards. Reasons to select two hard faces include.
- The sealed product includes abrasives and will wear the carbon face rapidly, e.g. gypsum slurry, sea water, crude oil, etc.
- The sealed product has a high viscosity and will tend to bond faces together, e.g. bitumen.
- On applications where no contamination from carbon wear debris is acceptable, e.g. pharmaceutical, food, make up, etc.
- On some de-ionised water applications the carbon can ‘wear’ rapidly.
- The duty temperature is higher than that tolerated by carbon graphite.
- When the carbon face will be attacked by the product e.g. acids.
If you’re looking for better service and support, there are some good mechanical seal specialists out there. Below are a few basic requirements that you should consider to determine if the company you are looking at has the ability to repair your seals.
- Do they have long standing experience repairing and supporting mechanical seals?
- Do they have the expertise and capability to repair and upgrade a wide range of seals?
- Can they provide an application list and example case studies?
- Can they provide references from a range of reputable customers?
- Do they operate according to an accredited quality management system such as ISO9001:2008?
- Do they work to the appropriate industry standards?
- Do they have IEC 60079-19 Certified Service Facility status?
- Do they use a fully integrated business system?
- Is their supply chain supported by companies that operate to the required industry standards?
If the company ticks all or most of these boxes then arrange a visit to meet the people and have a look at the facility to complete the picture.
Submit your question and we will respond as soon as possible.