balanced vs unbalanced mechanical seal price

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.

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.

The balance ratio of a mechanical seal is an area ratio and is related to the seal face load. Balance ratio is defined as the ratio of the closing area to the opening area. Seals with a balance ratio > 1 are "unbalanced"; ratios of < 1 are considered "balanced". Seals are balanced to decrease friction and wear, so you will usually move to a balanced seal at pressures of 250 psid and above (although you can use them at lower pressures as well). Unbalanced seals will be used up to 250 psid. The theory becomes more complex when you talk about metal bellows style mechanical seals; in those designs the balance ratio will increase at higher pressures primarily due to the bellows plate geometry. Visually if you were to look at the primary face of an unbalanced pusher style seal vs. a balanced pusher style seal you would notice a "step" in the face geometry (reducing the closing area and changing the balance ratio).

In theory, the lower the balance ratio, the lower the fluid film temperature, the longer the seal life. In practice, leakage control can sometimes be sacrificed and the faces may become unstable depending on if the fluid is volatile (vaporizing), or if any other face distortions occur. In general, review of the application with your seal vendor is always recommended. If you are adhering to API 682; then balanced seals are your only options due to the nature of the services. The attached is a very rough representation of what I just said. It depicts a single set of seal faces pressurized from the OD: the balance ratio calculations will become more complicated when you discuss OD vs. ID pressurizations particularly in dual pressurized seals. Hope this helps.

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.

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.

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.

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.

The inception of Globe star engineers (india) Pvt. Ltd. was in the year 2011, by Mr. Dhiraj C. Siddhapura with a mission to cater the industry with the international quality products with a very economical price. We started to step up the ladder of the success with his guideline of the huge experience and intense knowledge. He coordinates and manages the things wisely, which make us able to acquire the eminent position among the most trustworthy Manufacturers & Exporters and service Provider of the various types of mechanical seals of India.

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.

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.

We serve Single Spring Balanced Seal to the exact place where it requires. The rotating shaft with medium speed and higher pressure required these types of seals. As the design covers the spring part it can be considered as non-clogging of spring area, so that we can use these seals with sludge and waste and scraps contaminated medium. And the rotary assembly is driven by screw lock these seal is independent to direction, which can be used for double direction applications.

 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.

After completing a recent training class, I had opportunity to ask our customer what were some of the highest cost failures they experienced. The answer? Mechanical seal failures. Mechanical seals come in a wide variety of configurations and manufacturers. The cost of these seals can range from $1000 to $3000 per inch of shaft diameter. These are very close tolerance and will not withstand misalignment for long if at all. A high percentage of mechanical seal failures are due to vibration induced by misalignment.

While researching several mechanical seal manufacturers to gain some insight as to what their tolerances were (they are specific to configuration and are provided with the mechanical seal), I ran across the following very good article on mechanical seal basics.

Years ago, most pump shafts were sealed using rings of soft packing, compressed by a packing gland, but this type of shaft seal required a fair amount of leakage just to lubricate the packing and keep it cool. Then came the development of the “mechanical seal,” which accomplishes the job of restraining product leakage around the pump shaft with two very flat surfaces (one stationary and one rotating). Even though these mechanical seal faces also require some (very small) leakage across the faces, to form a hydrodynamic film, this leakage normally evaporates and is not noticeable. Most pump shafts today are sealed by means of mechanical seals. However, because of the delicate components used for this new sealing method, mechanical seal failures are the greatest cause of pump down time. This begs for a better understanding of this seal type and its application.

Mechanical seals are leakage control devices, which are found on rotating equipment such as pumps and mixers to prevent the leakage of liquids and gases from escaping into the environment. Figure 1 above shows a typical centrifugal pump, which highlights its constituent parts, including the mechanical seal.

A mechanical seal consists of 2 principle components. One component is stationary and the other rotates against it to achieve a seal (Figure 2). There are many types of mechanical seal, ranging from simple single spring designs to considerably more complex cartridge seal types. The design, arrangement and materials of construction are essentially determined by the pressure, temperature, speed of rotation and product being sealed (the product media).

III. The seal between the rotating member and shaft or shaft sleeve (4). This is known as the secondary seal and may be an o -ring as shown, a v -ring, a wedge or any similar sealing ring.

3 of the 4 main sealing points need little explanation, but consideration is required for the sealing point between the rotating and stationary components (faces). This primary seal is the basis of a mechanical seal design, and is what makes it work. The rotating component (3) and stationary component (1) are pressed against each other, usually by means of spring force.The mating faces of both components are precision machined (lapped) to be extremely flat within 2 light bands, which is an optical method of measuring flatness).

Spring compression (usually) provides initial face pressure. This pressure is maintained when the seal is at rest via the spring(s) thus preventing leakage between the faces

If the mechanical seal faces rotated against each other without some form of lubrication they would wear out (and the seal would fail) due to face friction and the resultant heat generated. So, lubrication is required which for simplicity, is supplied by the product media. This is known as fluid film and maintaining its stability is of prime importance if the seal is to provide satisfactory and reliable service.

The primary disadvantage of this seal type is that it is prone to secondary seal hang-up and fretting of the shaft or sleeve, especially when the seal is exposed to solids. A pusher seal type should not be selected if the secondary seal is likely to hang-up. Can small deposits of solids form ahead of the secondary sealing member?

There are multiple designs available for the mechanical seal configuration. Understanding how they work will help the readers select the appropriate type for their application.

NON-PUSHER OR BELLOWS SEAL does not have a secondary seal that must move along the shaft or sleeve to maintain seal face contact. In a non-pusher seal the secondary seal is in a static state at all times, even when the pump is in operation. A secondary sealing member is not required to make up the travel as the rotary and stationary seal faces wear. Primary seal face wear is typically accommodated by welded metal or elastomeric bellows which move to assist in the compression of the rotary to stationary seal faces.

The advantages of this seal type are the ability to handle high and low temperature applications (metal bellows), and that it does not require a rotating secondary seal, which means it is not prone to secondary seal hang-up or shaft/sleeve fretting. Elastomeric bellows seals are commonly used for water applications.

The disadvantages are that thin bellows cross sections must be upgraded for use in corrosive environments, plus the higher cost of metal bellows seals.

CARTRIDGE SEALS have the mechanical seal pre-mounted on a sleeve (including the gland). They fit directly over the shaft or shaft sleeve, and are available in single, double, and tandem configurations. Best of class pump users give strong consideration to the use of cartridge seals.

The advantages are that this seal configuration eliminates the requirement for seal setting measurements at installation. Cartridge seals lower maintenance costs and reduce seal setting errors.

Single seals do not always meet the shaft sealing requirements of today’s pumps, due to the small amount of required leakage when handling toxic or hazardous liquids; suspended abrasives or corrosives in the pumpage getting between the seal faces and causing premature wear; and/or the potential for dry operation of the seal faces. To address these situations, the seal industry has developed configurations which incorporate two sets of sealing faces, with a clean barrier fluid injected between these two sets of seal faces. The decision to choose between a double or single seal comes down to the initial cost to purchase the seal vs. the cost of operation, maintenance and downtime caused by the seal, plus the environmental and user plant emission standards for leakage from the seal.

The more common multiple seal configuration is called a Double (dual pressurized) seal, where the two seal face sets are oriented in opposite directions. The features of this seal arrangement are:

The metal inner seal parts are never exposed to the liquid product being pumped, which means no need for expensive metallurgy; especially good for viscous, abrasive, or thermosetting liquids.

The other multiple seal configuration is called a Tandem (dual unpressurized) arrangement, where the two individual seals are positioned in the same direction. This seal arrangement is commonly used in Submersible wastewater pumps, between the pump and motor, with oil as the barrier liquid. The typical features of this seal arrangement are:

The proper selection of a mechanical seal can be made only if the full operating conditions are known. Identification of the exact liquid to be handled is the first step in seal selection.

For best results with double (or tandem) seals handling abrasive, the inboard seal faces should be a hard material, such as silicon carbide vs. silicon carbide, while the outboard seal faces should have maximum lubricity, such as silicon carbide vs. carbon graphite.

The seal type and arrangement selected must meet the desired reliability, life cycle costs, and emission standards for the pump application. Double seals and double gas barrier seals are becoming the seals of choice. Finally, it should be noted that there are special single seal housing designs that greatly minimize the abrasives reaching the seal faces, even without an external water flush, but this is a subject for another column.