DFT check valve sizing-common-mistakes-ebook

Common Mistakes in Check Valve Sizing

DFT Valves

DFT Inc. 140 Sheree Blvd. PO Box 566, Exton, PA 19341

toll-free 800-206-4013 fax 610.524.9242 e-mail [email protected] 2

Check valves play a vital role in fluid handling systems; selecting the correct valve requires knowledge of the

process and components involved. The valve must be able to withstand the environmental conditions of its

specific application. Also, the components of a system are limited to the sizes available, and often have a higher

capacity than the system requires.

Therefore, one of the most important considerations is valve sizing. The most appropriate-sized valve must

be picked in order to render the best service. An undersized valve will cause higher pressure losses and create

excessive noise and vibration, and an oversized valve can lead to a premature wear and failure of the valve’s

internal components. Choosing the best valve will ensure proper flow, optimize overall efficiency and enhance

the integrity and longevity of any fluid handling system.

Introduction



There are a number of mistaken assumptions that can lead to oversized valves. It is important not to assume

that the line size and the valve size are the same; valves should be sized for their application, not their line size.

This is especially important for check valves, which are flow sensitive.

The flow conditions of check valves determine the quality of their internal performance and longevity of the

valve. This is because the disc inside a check valves is always in the flow stream, unlike other on/off valves

like gate, plug or ball valves. Generally, on/off valves are chosen based on the line size and the largest Cv

(flow coefficient) possible to minimize pressure losses of the entire system. When check valves or other flow

sensitive valves are chosen this way, it will lead to oversizing of the valve.

A check valve functions by allowing the flow to go in one direction and automatically preventing back flow

when the fluid stops or reverses its direction. The internal disc moves automatically to allow the flow of liquids,

gases, steam, and other media to pass forward, opening the valve. Then, as the forward flow decreases or is

reversed, the disc begins to close. Check valves are self-actuated and do not need assistance to open and close,

meaning that they will work without electricity, air, or human intervention.

If a check valve is purchased to accommodate the largest capacity possible, it could suffer from not having

enough flow in other operating scenarios. Without sufficient flow, the disc inside the valve, because it is in the

flow path, will constantly move because the forces exerted by the fluid are not sufficient to keep the valve in a

full open and stable condition.

When systems are designed, the most adverse possible conditions should always be considered. Extreme

weather, corrosive conditions, heavy loads, and other factors are all weighed in order to make the most reliable

fluid handling system. Components of a fluid handling system often have a greater capacity than the system

requires. There are many good reasons to do this, future capacity considerations, upset or accident scenarios

are a few examples. Under these conditions, it is imperative to choose correctly sized valves for the specific

application. However, the success of a check valve will depend on the actual conditions of the system, not the

most extreme.

Common Mistakes that Lead to Oversizing



There are a number of adverse consequences that come from the implementation of oversized valves. One of

the primary factors to consider is excessive valve leakage. In many valve standards maximum allowable leakage

is expressed as a fixed leakage rate per NPS (Nominal Pipe Size) of the valve. In this way a 12” NPS valve is

allowed 20% more leakage than a 10” NPS valve.

Excessive leakage is just one of the risks associated with oversizing. Oversizing of check valves can also result in

unreliable valve performance. Pressure drops are calculated with the assumption that the check valve is 100%

open, which is not the case when an oversized check valve is installed. Therefore the actual installed pressure

losses due to the valve selected will be greater than pressure losses calculated based on a fully open valve.

Unreliable performance can have a major impact, as check valves are used in many critical services in many

industrial applications including automotive, aviation, hospitals, water, wastewater, chemical plants, power

plants and many other industries.

The poor valve performance associated with oversized valves can be attributed to the advanced wear of the

individual components of the valve. Insufficient flow in oversized valves will result in increased metal-on-metal

wear of its disc, but many other components are affected as well, including stems, hinge pins, springs, internal

stops to mention a few. The flutter associated with flow instability can cause internal guides and bearings to

wear quickly. In certain cases, the check valve’s disk could open for a very short period of time and then bounce

on and off its seat continuously, which causes rapid deterioration of all metal components within the valve. The

resulting damage that ensues will eventually lead to poor valve performance as observed by excessive through

leakage or valves stuck open.

In extreme cases, oversized valves can result in the complete failure of valve components. Maintenance

inspections have been known to discover instances where internal components are entirely missing. This failure

can result in detrimental back flow, downstream pressure loss, and in some cases, total system malfunction.

Any time a check valve is missing components, it cannot properly protect the pump or whatever components

are located downstream of the valve. All valves and equipment within the system could be in danger of sudden

failure. These sudden failures could include pump shaft failures,

impeller failures, back leakage throughout the system or even

external spills impacting workers and the environment.

Risks of Oversizing



Choosing the appropriate sized check valve means it will be working at its highest level, providing efficient

service with maximum protection. Proper check valve sizing will optimize a system’s reliability, providing the

longest and most trouble free service. When the valve’s disc is stable and in the fully open against the internal

stop or fully closed position against the seat, no disc fluttering will occur.

This results in reduced overall costs. Not only do the valves last longer, but pumps and other related

components on the same system will have increased longevity as well. Well-functioning check valves also

enhance the safety of their applications. Check valves are made in a variety of types and styles and are used for

all sorts of applications. The following are a few examples:

Ball Checks: These valves control the movement of flow by using a “ball” inside the body. This ball can rotate

freely, which results in even wear and a wiping action that makes it very useful for handling viscous media.

Dual Plate: Consisting of two spring-loaded semicircular plates hinged in the center, these valves have

excellent flow capacity. They are lightweight and feature compact face-to-face dimensions. Their compact size

means they need less support and mechanical room.

Piston Check: Available in T pattern and Y pattern body designs, these valves feature a body-guided or stem

guided disc that moves within the body bore. This ensures the alignment of the disc and seat when the valve

closes. Generally Y pattern bodies have a higher flow coefficient than the T pattern bodies.

Swing Check and Tilting Disc: These valves are designed with a hinged arm at the top of the valve. The

valve is closed with the assistance of reverse flow and gravity. They are available with socket weld, threaded,

flanged, ring type joint or butt weld end connections.

Spring Assisted In-Line / Axial Flow Check: Sometimes

referred to as “Silent Check Valves” or “Nozzle Check” valves, these

valves are designed to prevent reverse flow and reduce or eliminate

water hammer. Using a spring-assisted disc that has been constructed

in line with the flow with a short travel distance, this is a fast-closing

valve. It can be installed in any position, in either vertical or horizontal

pipe runs and is available in a number of different styles and end

configurations.

The Importance of Accurate Check Valve Sizing



Water hammer is a serious problem that can in piping systems with a liquid medium. It is caused by shock waves

generated by relatively incompressible liquids being stopped abruptly in a pipe by a sudden valve closure or

sudden pump shutdown. The risk of water hammer always exists when there are incompressible fluids (liquids

and slurries) in a piping system, and it runs especially high under the following circumstances:

• When the fluid is running at a fast velocity

• When the piping system features large elevation changes

• Vertical piping runs right after a pump

• Long horizontal piping runs

• A swing check valve installed in a vertical, flow up piping system

The swing check relies on gravity and/or fluid flow in order for it to close, so flow reversal needs to happen

before the valve begins closing. When the closure of the swing check occurs, it will often abruptly stop the flow,

causing a large pressure surge that creates shock waves within the fluid. These shockwaves continue to occur

until the energy is dissipated through internal fluid friction and dampening.

Water hammer can have a number of damaging symptoms, including vibration, noise, and hammering sounds

that can result in some serious structural problems. Damage in the form of: equipment internal component

damage, flange leakage, pipe rupturing, and damage to pipe supports and structure. These shock waves can

exert very strong forces against piping systems and valves, often 5 to 10 times the normal working pressures of

the system causing potentially damaging stress or failures. Normally systems are designed for a factor of safety

of 1.5 or 2.5, but they are not designed to withstand the forces that these shockwaves can create.

A properly functioning and appropriately sized spring-assisted non-slam check valve can eliminate or deeply

minimize water hammer and its associated problems. These valves do not rely on fluid flow or gravity for their

closure. The spring assist on the valve begins to close the disc as the forward velocity of the fluid begins to

slow. Once the fluid pressure on the disc becomes less than the pressure exerted by the spring, the valve will

begin to close.

The disc only needs to travel a relatively short distance; this, along with the

spring assist, means that by the time that the forward velocity has decreased

to zero, the valve is closed because the disc has already reached the seat. This

eliminates reverse flow, therefore virtually eliminating the forces that create

water hammer.

Preventing Water Hammer



It’s important for all buyers to understand the importance of purchasing the most appropriately sized check

valves, but nothing can replace talking with a professional. Over the years, many different styles of check valve

have been designed and developed to meet a variety of customers’ needs. The right professional can help solve

problems that check valves might encounter and prevent as many failures as possible. Professionals can give

the best assistance in a variety of areas regarding check valves.

When determining the exact size for a check valve, for instance, it is sometimes not possible to fit a 6” valve into

an 8” system. A professional may be able to modify the internals of the valve in order for it to provide the most

appropriate flow characteristics for the conditions of the system.

A professional can also help determine the strength of a spring for an in-line check valve. Stronger or weaker

designs may be needed in order to achieve the optimum closure speed or cracking pressure for applications

with a very low flow. There may be additional enhancements available that a professional valve manufacturer

can recommend for a specific application. Low friction bushings and special coatings, for example, may cost

extra money upfront, but can be highly beneficial for systems to run most efficiently.

Talk to a Professional for the Best Valve Fitting



Every fluid handling system has its own unique requirements. When customers purchase inappropriately sized

valves, it can create costs that go far beyond the valves themselves. When a valve fails, the labor costs to fix

it can be very high. For example, if only two people are sent to replace a valve, it will generally cost in the

neighborhood of $100 per hour for the crew alone.

When you add in lost production time on top of that, the costs can be staggering—sometimes ranging up to

$10,000 per hour or higher. A professional consultation to get the most appropriate and best-sized valve is far

more cost effective when considering lifetime maintenance costs, system reliability and optimizing system

performance through maintenance reductions.

The price of poor valve choice could be even higher when working with potentially hazardous materials. A valve

malfunction could create a spill that could harm the environment. Most chemical spills need to be reported,

impacting environmental safety, worker safety, and damage to the reputation of the company as well as the

potential for large fines.

In North Dakota, for instance, the smallest oil spills create massive amounts of paperwork and remediation

costs. Even seemingly non-hazardous spills such as water at a drilling site can lead to difficulties for a

company’s operations and logistics. Creating the most optimally functioning system up-front can prevent or

minimize these events, saving companies time, money, and all the deleterious effects of hazardous waste spills.

Lower Lifetime Maintenance Costs



There are a number of factors to consider when purchasing valves, and proper sizing is one of the most

important. A properly sized valve will prevent malfunctions and optimize any system’s overall reliability. Taking

the extra time to consult a professional to help choose the optimal check valve will pay off when it comes to the

bottom line.

It prevents the rapid wear of internal components while enhancing safety, both for the people who work with

the valves and for the environment. Considering the central role that valves play in a fluid handling system,

properly sized valves are essential. The best valve choice will ensure that companies get the most out of their

systems and equipment, today and far into the future.

Proper Sizing Ensures the Best Fluid Handling

DFT check valve sizing-common-mistakes-ebook

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