Difference between Zero Offset and Double Offset Butterfly Valve

Choosing the wrong butterfly valve for your process can have disastrous effect at overall operation. To avoid this situation we should know about difference between zero Offset and Double Offset Butterfly valves. Not only being specific about these two types but all of the butterfly valves, their applications, types, and components. Different process conditions demand different type of butterfly valve. Before diving deep into the applications and differences let us know basic terminologies, components and their roles in live and dead process.

Basic Terminologies and their Role

Butterfly Valve

A butterfly valve is a quarter turn (90°) rotational motion valve, which is used to stop, regulate and start flow in a process. Butterfly valves are famous for less space utilization and easy & fast operation.

Components

Valve Body

Part of butterfly valve fit between pipe flanges. It has many designs depending upon application but most common are Lug and wafer body type. The best advantage of butterfly valve is width of valve body that is quite thin as compared to other valve types.

Valve Disc

Function of this part of valve is to stop, regulate and stat flow in process. It is equivalent to plug in a plug valve and gate in gate valve. There are several variations in disc design relating to flow rate and application.

Disc Seat

Disc seat is packing between valve body and valve disc. It also provides tight seal to avoid leakage. There are various designs of disc seat depending upon material and valve geometry. Common disc seats are dovetail, catridge and bonned.

Valve Stem

Stem is responsible for motion of valve disc. It rotates manually by handle at outer part of stem or electronically with utilization of gearbox.

Stem Bearings

Stem bearings provide support to stem both at top and bottom part of stem. It also ease move of stem and disc.

Other Components

Other components include retaining rings, manual rotation handle, gland packing, gland studs and nuts, stem packing, thrust ring and bolts.              

Types of Butterfly Valves

Depending upon Assembly or Connection

Based on assemble requirements there are three types of butterfly valve commonly available. Following are the types:

Flange Butterfly Valves

These butterfly valves have a flange at each end. These flanges make connection with flanges of pipe for assembly. A gasket is used between each connection as per the recommendations given at Piping and Instrumentation drawing.

Lug type Butterfly Valves

This type of butterfly valves is an extension of flange butterfly valves. This style of valves have threaded metal inserts threaded in valve’s bolt holes. This allows connection with two sets of bolts with no nuts. Lug style valves serves applications having reduced pressure rating.

Wafer Butterfly Valves

Wafer butterfly valves is more common and less expensive of three.  Unlike flange and Lug style valves, it have only two holes at the top and two at the bottom for connection. To install a wafer style butterfly just place it between pipe flange and have a connection using bolts or studs with nuts.      

Figure 2 Butterfly Valve Types Based on Connection

Depending Upon Geometry

When focusing on geometry of butterfly valves we can categorize them as following chart depict.

Figure 3 Types Depending on Geometry

Zero Offset Butterfly Valves

Zero offset butterfly valves are concentric by geometry. As the name shows concentric means at center. In the past while, designing butterfly valves Engineers kept two things in mind: disc center and shaft center. Both centers met at a concentric pint. Moreover, there was not offset given from any of these centers. As shown in diagram below.

Figure 4 Butterfly Valve Offsets

These valves serves for generic purposes and can be ‘resilient seated or ‘rubber seated’. The stem and disc are connected in the seat of the valve. Pressure and temperature for zero offset butterfly valves are 200 PSI and 400 °F respectively. Applications where we can use zero offset butterfly valves include chemicals, coatings, food, some solids and abrasives.

Zero Offset Butterfly Valves are available in multiple materials but most commonly, stainless steel and epoxy coated cast iron with numerous seat materials are used in industrial applications. Seat Materials can be Teflon, BUNA, Natural Rubber, VITON, EPDM and others.  

Problems with Zero Offset Butterfly Valves    

Problems with Zero Offset Butterfly Valves or reasons to provide double or triple offsets are as following:

1. Disc Rubbing: When we rotate valve disc it touches seat which can cause rubbing and will lead to leakage
2. Seat Materials: mostly include rubber and low elasticity materials so can handle only low temperatures as shown below  

• Low pressure rating makes it too generic as compared to other quarter turn valves

However, as we know butterfly valves have advantages like less space utilization and easy operation, which is a big constraint in industry it needed some, design changes. Therefore, Engineers came up with the idea of having offset and manufactured eccentric valves. Eccentric valves are of two types as double offset butterfly valve and triple offset butterfly valve. Offset refers to being away of center where disc and shaft centerline meet. Details of both of these is as below:

Double Offset Butterfly Valve

Double offset Butterfly Valve is also known as high performance butterfly valve. It has two stem offset as compared to concentric butterfly valve. This will create a cam action during operation. Figure 4 depict two offsets given in double offset butterfly valve and we can describe as following:

1. Axis of the shaft is behind the centerline of sealing point of the disc to seat
2. Axis of the shaft is eccentric to the center of the valve or pipeline

Above given offsets will solve problem second and third but can still have first problem that is rubbing. However, rubbing in double offset butterfly is very nominal and we can solve this problem by changing seat material.

Valve and Seal Materials

Material to solve this problem should be a metal as disc is metallic and elastic to minimize the effect of rubbing. Materials should be elastomeric. So, that they return to shape after rubbing with valve disc. Materials should comply standard specifications of API 607. Double offset butterfly valves can reach pressure ratings of ANSI Class 600. Materials for making these valves can be stainless steel, carbon steel or Hastelloy with an elastomeric material. Hastelloy are nickel and molybdenum alloy and for corrosive applications nickel chromium and molybdenum alloy.   

Working Mechanism

As double offset butterfly valve is eccentric disc moves away from seat as it begins to open. For an effective bubble, tight sealing this valve utilizes line pressure that acts against the disc. Temperature and pressure range for double offset butterfly valves is 572°F and 300 PSI respectively.       

Applications

Applications for Double Offset Butterfly valve include industries such as oil and gas, refining, chemical and petrochemical applications, pulp and paper, offshore applications, water and wastewater treatment, HVAC, power generation and so on. Double Offset Butterfly is primarily preferred where bubble tight sealing is required. Because with elastomeric seat material and metallic disc it becomes leak proof service. Avoid using it in dead pipelines or where long work stoppage.

Double Offset Butterfly Valve Disadvantage/Problem

• Despite high performance, elastomeric material, high temperature and pressure rating still seat rubbing can be an issue in abrasive and corrosive applications  

To solve issues relating to Zero and double offset butterfly valves Engineers introduced another valve called triple offset butterfly valve to avoid rubbing issue completely. To perform this task perfectly Engineers checked the geometry of gate valve. In gate, valve seats are slanted. Therefore, if we use same geometry on one side of double offset butterfly valve we can overcome rubbing issue. We can see gate valve geometry below:

You can observe that seat is slanted. Therefore, when valve is shut off it will completely rest against bottom. This concept is utilized in triple offset butterfly valve.

Triple Offset Butterfly Valve

It is a form of eccentric butterfly valve having three offsets. Figure 4 depict three offsets in this valve and we can illustrate them as:

• First Offset: This applied to the centerline of the disc/seat sealing surfaces, which provides and uninterrupted sealing surface.
• Second Offset: This offset refers to the location of the shaft with respect to centerline of the bore, which enables caming action of the disc to minimize rubbing and wear between seal and seat.
• Third Offset: This offset is the axis of the seat cone angle that is offset from the centerline of the valve bore to eliminate rubbing of the seat/seal contact surfaces during operation and to preserve sealing integrity over the life cycle of valve.

Working Mechanism

Its working mechanism is not as the typical butterfly valves, which with turn of lever move sideways to allow media entry like waving of a hand. Despite it acts like a globe valve where disc moves somewhat like a cone as moving upward to downwards.   

Valve and Seat Material 

The triple offset butterfly valves usually deal with abrasive and corrosive media so valve often has metal seats made from high-grade materials such as stainless steel or duplex steel. These valves can also have double seat type design that is an elastomeric or Teflon covers a stainless steel seat. Seats of this valve are field replaceable. However, this should only be employed where media in not abrasive of high temperature.

Applications

The triple offset butterfly valve has numerous applications. These valves are useful in special challenging environments including the abrasive materials, high temperature, pressure, and even those prone to fugitive emissions. Most common examples are pure steam or water lines. Industries include wastewater treatment, chemical slurry lines, power generation and HVAC. Triple offset butterfly valves are expensive so prefer only in critical applications.

Summary

We can summarize Zero, Double and Triple Offset Butterfly Valves as in table below. Here we only listed most common features for comparison and as guide for choosing the right valve.

Conclusion

Well if your application is generic not involving any corrosive or abrasive material you can use a zero offset butterfly valve. For industrial applications go for high performance or double offset butterfly valve. For critical applications and challenging environments, a triple offset butterfly valve is worth considering.