balanced vs unbalanced mechanical seal in stock

A. A balanced seal is a mechanical seal configuration in which the fluid closing forces on the seal faces have been modified through seal design. Seal balance, or balance ratio of a mechanical seal, is simply the ratio of two geometric areas. These areas are called the closing area (Ac) and the opening area (Ao). The closing area is different when pressure is on the outer diameter of the seal than when the pressure is on the inner diameter. When the pressure is on the outer diameter, the closing area is from the seal face outer diameter down to the lowest point, where the secondary seal rests against the shaft or sleeve. When the pressure is on the inner diameter, the closing area is from the highest point, where the secondary seal rests against the primary ring counter bore, down to the sleeve diameter.

The opening force is always the area of the sealing faces. The balance ratio is then Ac/Ao. A seal with a balance ratio less than 100 percent is called a balanced seal. A seal with a balance ratio greater than 100 percent is called an unbalanced seal. Most balanced seals have a balance ratio between 60 and 90 percent. Most unbalanced seals have a balance ratio between 110 and 160 percent.

Pusher seals normally require a step in the shaft/sleeve or internal hardware to achieve a balanced design. Metal bellows seals do not require this step. The balance diameter, or mean effective diameter (MED), of metal bellows seals is located near the middle of the convolution. When pressure is applied to the outer diameter of the seal, the MED shifts downward, lowering seal balance. The opposite is true when the seal is subject to internal pressure. The rate of change in the balance depends on the face width and the bellows leaflet design.

Pusher seals can be designed to withstand pressure from either direction. This is accomplished by trapping the O-ring between two diameters as shown in Figure 4.1. The cavity must be long enough to allow the O-ring to move, allowing pressure to act on the primary ring. These designs allow the seal to withstand system upsets.

The balance of a seal refers to the distribution of load across the seal faces. If there is too much load on the seal faces, it can lead to a leakage of fluids from within the seal which essentially renders the seal useless. Moreover, the liquid film in between the seal rings runs the risk of vaporising.

This can result in higher wear and tear off the seal, shortening its life span. Seal balancing is therefore necessary to avoid disasters and to also elongate the life of a seal.

A balanced seal has a higher pressure limit. This means that they have a larger capacity for pressure and they also produce less heat. They can handle liquids that have a low lubricity better than unbalanced seals.

On the other hand, unbalanced seals are typically much more stable than their balanced counterparts as far as vibration, cavitation and misalignment are concerned.

The only major drawback that an unbalanced seal presents is a low pressure limit. If they are put under even slightly more pressure than they can take, the liquid film will quickly vaporise and will cause the running seal to run dry and thus fail.

Many early mechanical seal designs placed the spring inside the process fluid. Most products (process fluids) that are sealed are not very clean. When the spring mechanism of the mechanical seal is immersed in this unclean fluid, dirt collects between the springs. This situation eventually impacts the spring’s ability to respond to movements and vibrations, and the ability to keep the seal faces closed. Over time, clogging of the springs will cause premature seal failure.

The ideal design offers springs on the atmospheric side of the mechanical seals. The springs will be protected from the process fluid and their ability to work will not be impeded.

The pressure from both the seal springs (Ps) and the hydraulic pressure of the liquid in the pump (Pp) provide a compression force that keeps the seal faces closed. Balanced seals reduce the seal ring area (Ah) on which the hydraulic pressure of the liquid in the pump (Pp) acts.

By reducing the area, the net closing force is reduced. This allows for better lubrication that results in lower heat generation, face wear, and power consumption. Balanced seals typically have higher pressure ratings than unbalanced seals.

Mechanical seals can be designed with inserted seal faces or with monolithic seal faces. In both cases, the sacrificial seal face is often made from carbon/graphite. This material offers good running properties but is relatively weaker from a mechanical standpoint than other options. Inserted face designs use a metal rotary holder to transmit the shaft torque to the seal face.

The disadvantage of this inserted face design is that the face and holder material have different coefficients of thermal expansion. This changes the net interference force between both parts when they are exposed to heat from the process fluid or face friction. The seal face deforms, which results in leakage and accelerated wear.

More modern seals are equipped with monolithic seal faces that are made out of only the seal face material itself. The torque transmission is applied directly to the seal face. This is possible if the geometry of the seal face is designed in a particular shape to give it the strength to handle the torque through its geometric design. These monolithic seal face designs have been made possible through the use of Finite Element Analysis (computer modeling).

Monolithic seal faces provide a more stable fluid film between the faces, and they do not deform in operation compared to inserted faces (or to a much lesser degree). Therefore, they are more commonly used nowadays when reliability and low emissions are vital.

All mechanical seal designs have at least one secondary seal that interacts with the dynamic movement of theflexible mounted face. This secondary seal moves with the springs to keep the seal faces closed and is defined as thedynamic secondary seal. During operation of a rotary design, springs will keep the seal faces closed. They adjust with each rotation for any misalignment from installation and parts tolerances. As the springs compensate, the dynamic secondary seal moves back and forth, twice per revolution. This rapid movement prevents the protective chrome oxide layer (the layer that protects the metal) from forming. Erosion of this unprotected area under the dynamic secondary seal will cause a groove to develop. Eventually this groove becomes so deep that O-Ring compression is lost and the seal leaks. In most cases, fretted shafts must be replaced to achieve an effective seal.

With rotary mechanical seals, it is important that the stuffing box face is perpendicular to the shaft for the faces to stay closed. There will always be some resulting misalignment from installation and parts tolerances. The springs must adjust with each rotation to keep the seal faces closed. This adjustment becomes more difficult at higher speeds.

In contrast, a stationary seal is a mechanical seal designed in such a way that the springs do not rotate with the pump shaft; they remain stationary. Because the springs do not rotate, they are unaffected by rotational speed. The springs do not need to correct or adjust with each rotation; they adjust for misalignment only once when installed.

Rotary seals are simple in design which makes them inexpensive. They are suitable for lower speeds only. Stationary seals are more complicated to design but are suitable for all speed ranges. Because of design complexity, stationary seals are more commonly configured as cartridge seals rather than component seals.

Marco Hanzon is Vice President of Global Marketing for A.W. Chesterton Company. He has been an active member and past chairman of the Mechanical Seal Committee of the European Sealing Association. Marco"s experience includes working as an In-Field Support Engineer for mechanical seals.

Mechanical seals have classified several types. In this article, we will see the basic classification of mechanical seal that is the “Mechanical Seal – Balanced and Unbalanced Type”.

The pressure in any stuffing box acts equally in all directions and forces the primary ring against the mating ring. The force (F) acts only on the diameter (Do) across the seal face, it acts as a closing force on the seal faces.

To relieve the force at the seal faces, the diameter of the shoulder on a sleeve or the seal hardware is decreased. Thereby the seal face pressure can be lowered. This is called seal balancing.

A seal without a shoulder in the design is an unbalanced seal. A balanced seal is designed to operate with a shoulder. Only metal bellows seal is a balanced seal that does not require a shoulder.

Virtually all mechanical seals are available in either unbalanced ( Ref. Figure) or balanced versions. The term “unbalanced” is used when the stuffing box pressure times the area exposed to the pumped fluid (closing force), acting to close the seal faces, is greater than the average pressure between the seal faces (pressure gradient)times the area of contact between the faces. In other words, unbalanced mechanical seal exhibit net hydraulic closing forces which are generated by the actual pressures to be sealed.

For example, if there were a stuffing box pressure of  50 psig (3.4barg), the spring load would have to be added. Hence, the “face load” or closing force on the faces would be even higher than 50 psig times the face area. This, of course, limits the pressure sealing capacity of an unbalanced seal.

Unbalanced seals are often more stable than balanced seals when subjected to vibration, misalignment and cavitation. The disadvantage is their relatively low-pressure limit. If the closing force exerted on the seal faces exceeds the pressure limit, the lubricating film between the faces is squeezed out and the highly loaded dry running seal fails.

The balanced seal has the same opening (face) area as the unbalanced seal, but the closing area has been reduced about the face area. Because force equals pressure times area, reducing the closing area reduces the closing force. Consequently, less heat is generated and the seal generally has a longer life.

To simplify the explanation, balancing mechanical seal involves a small design change which reduces the hydraulic forces acting to close the seal faces. Balanced seals have higher pressure limits, lower seal face loading, and generate less heat. They are better able to handle liquids with low lubricity and high vapour pressures. This would include light hydrocarbons. Because seal designs vary from manufacturer to manufacturer and from application to application, it is not possible to standardize on either configurations or materials that cover all conceivable services. Available basic designs have variations that were often developed to meet specific applications. Each seal design has its own strengths and weaknesses.

Nowadays most of the seal manufactures are used only balanced mechanical seal. In some special mechanical seals (ie., engineered seals) are designed with unbalanced mechanical seal.

Balanced mechanical seals are more preferred than unbalanced mechanical seals. Seal balance can range from 0.65 to 1.35, depending on operating conditions.

Centrifugal pumps are one of the most extensively used pumps in municipal and complex industrial applications. However, a proper sealing arrangement is imperative for these pumps to prevent fluid leakage and protect the pump’s inside from contaminants in the atmosphere. Mechanical seals are preferred for sealing the pump as they require less maintenance and are much more durable than packing seals.

There are a variety of options in the market when it comes to mechanical seal systems. Before illustrating the types of mechanical seals for centrifugal pumps, here are four key considerations when choosing the appropriate seal system.

Consider the type of fluid that will be pumped and how it will affect the seal system design—factors such as lubricity, volatility, corrosive properties, and cleanliness matter the most.

Make a choice depending on the pressure exerted on the seal face. For instance, unbalanced seals are suitable for low-pressure applications, while balanced seals are appropriate in high-pressure conditions.

Temperature considerations will help determine whether you need to choose a pump with heat-sensitive components. For example, balanced seals sustain high temperatures better than their unbalanced counterparts.

These types of mechanical seals are typically low in cost and used for more generalized purposes. However, installing and adjusting standard component seals is time-consuming and requires a fair amount of operational skills. In addition, as they are composed of separate dynamic and stationary components, incorrect installation remains the major cause of errors.

Cartridge type mechanical seals are easy to install and ensure high performance. They are a one-piece unit incorporating all sealing components into a single assembly. Cartridge seals provide substantial maintenance advantages compared to other seal types while reducing installation time and the risk of assembly errors.

Pusher seals rotate along the shaft or sleeve to maintain contact with the faces of the seal to reduce wearing and wobbling caused by any misalignment. They are less expensive and come in different sizes and designs. The only drawback is that the elastomer is subject to wear.Non-pusher type seals maintain contact with the faces without rotating axially. They function under low temperatures and high pressures. However, the bellows used in these seals must be replaced frequently to work in corrosive environments.

Balanced mechanical seals work at high operational pressures while generating lesser heat. They are suitable for handling low lubrication liquids and high vapor pressure. Balanced seals increase seal life by reducing the closing force.Unbalanced mechanical seals are a more economical alternative that works for low/medium pressure applications. They are highly stable and still work in conditions where there are vibrations, shaft misalignments, or fluid cavitations.

 Here we describe the difference between a balanced and unbalanced mechanical seal. The mechanical seal is the most important and most used part. If you check its specification and type, then it can be classified into balanced and unbalanced mechanical seals.

 Before we check their difference, let’s understand what mechanical seal balance is first. It is the load that acts across the seal faces. This load between the seal faces should be optimum.

 If the load is exceptionally high, then the liquid film gets oozed out. When it vaporizes, it causes unstable conditions. Because of the thermos-electrical instability, there is high wear and tear on the sealing surface.

When the seal is balanced, it avoids this situation and leads to reduce energy consumption. It extends the life of the seal. The pressure in a stuffing box is applied equally in all directions. This pressure keeps the primary ring stable against the mating or rotating ring.

The face area or opening area of a balanced seal is the same as an unbalanced seal. But the closing area is maintained in proportion to the face area. When the closing area reduces, the closing force also reduces proportionately. Thus, less heat is generated, and the seal has an extended life.

Mechanical seals are critical components in centrifugal pump systems. These devices preserve the integrity of the pump systems by preventing fluid leaks and keeping contaminants out. Mechanical seal systems are used on various seal designs to detect leakage, control the seal environment and lubricate secondary seals.

Depending on the pump type and the process variables, there are various mechanical seal types to choose from. Each seal variant has its unique design and characteristics which make it suitable for a specific application. MES has years of experience with industrial mechanical seals and support systems, making us an authority in this area.

Mechanical seal types vary in design, arrangement, and how they disperse the hydraulic forces acting at their faces. The most common seal types include the following:

Balanced mechanical seal arrangements refer to a system where the forces acting at the seal faces are balanced. As a result of the lower face loading, there is more even lubrication of the seal faces and longer seal life. Learn about our mechanical seal lubrication systems today.

Balanced mechanical seals are particularly suited to higher operating pressures, typically above 200 PSIG. They are also a good choice when handling liquids with low lubricity and higher volatility.

Unbalanced mechanical seal types are commonly employed as a more economical option to the more complex balance seal. Unbalanced seals may also exhibit less product leakage due to tighter control of the face film, but as a result can exhibit much lower mean time between failure. Unbalanced seals are not recommended for high pressure or most hydrocarbon applications.

Pusher seals utilize one or multiple springs to maintain seal closing forces. The springs can be in the rotating or stationary element of the mechanical seal. Pusher type seals can provide sealing at very high pressures but have a drawback due to the elastomer under the primary seal face that can be subjected to wear as the face moves along the shaft/sleeve during operation.

Non-pusher seals utilize a metal or elastomeric bellows to maintain seal closing forces. These seals are ideally suited to dirty and high temperature applications. Bellows seals are limited to medium/lower pressure applications.

Conventional seals are typically lower cost and often installed on general service equipment. These seals require higher operator skill to service as they installed as individual components.

Cartridge type mechanical seals incorporate all of the seal elements into a single assembly. This dramatically reduces the potential for assembly error and the time require for seal replacements. Learn more about the difference between cartridge and non-cartridge mechanical seals today.

When deciding on the type of seal system for a centrifugal pump, operators must choose according to their unique application. Failure to select the proper seal type can lead to loss of pump integrity, breakdowns and costly repairs. To avoid these undesirable results, all operators must consider the following factors before deciding.

The amount of pressure exerted at a mechanical seal’s faces has a significant effect on its performance. If a pump is to be operated at low pressures, an unbalanced mechanical seal will be suitable. However, in conditions where higher pressures are anticipated, balanced seals will prove a more reliable solution.

Balanced mechanical seals perform better than their unbalanced counterparts in conditions where the operating temperatures are higher than normal. Heat sensitive components are better preserved in balanced mechanical seals compared to other seal types.

As it goes for all types of machinery, operator safety is the top priority. The use of double mechanical seals in centrifugal pumps provides additional protection as they have increased sealing capacity and are generally more reliable.

As a means of quantifying the amount, or percent, of balance for a mechanical seal, a ratio can be made between the seal face area above the balance diameter versus the total seal face area. This ratio can also be expressed as the area of the seal face exposed to hydraulic closing force versus the total seal face area.

As a general rule of thumb, balanced seal designs use a balance ratio of 0.75 for water and non flashing hydrocarbons. For flashing hydrocarbons, which are fluids with a vapor pressure greater than atmospheric pressure at the service temperature, the balance ratio is typically 0.80 to 0.85. Unbalanced seal designs typically have a ratio of 1.25 to 1.35.

Balance diameter varies with seal design, but for spring pusher seals under outer-diameter pressure, it is normally the diameter of the sliding contact surface of the inner diameter of the dynamic O-ring; for spring pusher seals under inner-diameter pressure, it is normally the diameter of the sliding contact surface of the outer diameter of the dynamic O-ring; for welded metal bellows-type seals, the balance diameter is normally the mean diameter of the bellows, but this can vary with pressure.

Temperature control plays an important role in the success of a mechanical seal. Every seal generates heat at the seal faces. In some cases, heat soak from the fluid pumped should also be controlled. Heat soak is the heat transferred from the pump and pumped fluid to fluid in the seal chamber. For example, if a particular fluid must be maintained at 60 °C (140 °F) to maintain a satisfactory vapour pressure margin and the pump operating temperature is 146 °C (295 °F), heat would be transferred through the pump case into the seal chamber.

There are two types of seals, balanced and unbalanced seals. The pump seal manufacturer manufactures it considering the balanced ratio. The biggest advantage of unbalanced seals is they are not so costly, highly stable and allow no leakage of fluid from the pumping casing.

Historically, centrifugal pumps have usually been designed with packing seals. Packing seals are packed with a lubricated fibrous material that came into direct contact with the shaft, so flush water is necessary to cool and lubricate the shaft. Flush water has to be directed away from the process to prevent contamination, and care has to be taken to protect the bearing box from flush water that contaminates the oil, as well as to prevent the safety problem of water pooling on the floor.

Mechanical seals may have a higher initial cost, but they often save a great deal of operational cost depending on how much flush water a packing seal pump consumes.

Balanced Seals have a system where forces acting on both faces are balanced, so there is more even lubrication of seal faces. Balanced seals have a higher cost than unbalanced seals, and they tend to last longer. Unbalanced seals show less leakage and are more inexpensive, but they have a lower mean time between failures and are not recommended for high pressure applications.

Pusher seals use one or more springs to maintain sealing forces, while non-pusher seals use elastomeric or metal bellows. Pusher seals can be used at very high pressure applications, but they have an elastomeric seal that can wear. Non-pusher seals are ideal for medium/low pressure and high temperature or dirty applications.

Conventional mechanical seals are installed as components. Cartridge seals have all the seal elements contained within a single assembly, so they are quick to install and reduce the chance of installation errors.

Many industrial processes still use a packing seal for positive displacement pump applications. Packing seals, which are designed to allow some controlled leakage, can be a reliable method of preventing excessive leaks. However, stricter standards in industries like food processing are reducing the amount of acceptable leakage.

Single Spring Balanced & Unbalanced SealOwing to our industrial expertise, we are able to manufacture, export and wholesale premium quality Single Spring Balanced & Unbalanced Seal. With an aim to ensure that offered unbalance seals are able to stand tall on the expectations of patrons, we manufacture these using best grade material. Along with this, we keep in mind industry set quality standards while manufacturing these unbalance seals. Prior to dispatch, we make these unbalance seals pass a quality check so as to ensure their flawlessness.

We are a prominent Industrial Balanced and Unbalanced Single Spring Seal. Clogging type applications. Torque Transmission from Retainer Shell to Dynamic Ring is done through Drive Lugs. GLOBE STAR Make Balance and Unbalance Single Spring Seals are Single Helical Coil Spring Seals Developed for clogging type applications.

Secondary Seal: U71 & B71 : Viton, EPDM, Silicon, Aflas, EPR, Kalrez, TCV, U76 & B76 : GFT, PTFE, FEP, Grafoil, Hardware : SS 316, SS 304, Hast - C, Monel, Alloy -20

Our organization is one of the prominent names in the market engaged in providing Inside Mounted Balanced and Unbalanced Single Spring Seal. These seals are of U71 &U76, B71 & B76 series and are Single Helical Coil Spring Seals. Provided seals are suitable for dirty media & clogging type applications. Drive lugs help torque transmission from retainer shell to dynamic ring. In these seals all the parts are fasten together through a snap ring that aid convenient installation & removal. All the parts can be changed from U71 to U76 & B71 to B76 by interchanging only the dynamic ring and the secondary seal.

We manufacturer U71 &U76 and B71 & B76 series of Slurry Seal. Contact with us to get the best deal of Slurry Seal. We also provide Slurry Seal on customized base.

When deciding on the type of seal system for a centrifugal pump, operators must choose according to their unique application. Failure to select the proper mechanical seal type can lead to loss of pump integrity, breakdowns and costly repairs. To avoid these undesirable results, all operators must consider the following factors before deciding.

The amount of pressure exerted at a mechanical seal’s faces has a significant effect on its performance. If a pump is to be operated at low pressures, an unbalanced mechanical seal will be suitable. However, in conditions where higher pressures are anticipated, balanced seals will prove a more reliable solution.

Balanced mechanical seals perform better than their unbalanced counterparts in conditions where the operating temperatures are higher than normal. Heat sensitive components are better preserved in balanced mechanical seals compared to other seal types.

As it goes for all types of machinery, operator safety is the top priority. The use of double mechanical seals in centrifugal pumps provides additional protection as they have increased sealing capacity and are generally more reliable.

It is important to think about the mechanical seal in terms of its total lifetime costs – not so much by its initial cost. Start a reliability program that defines the cost of failure and justify it by increasing the mechanical seal’s mean time between failures.