The Difference between Nozzle and Silent Check Valves
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Why a Nozzle Check Valve Can be a Silent Check Valve but a Silent Check Valve can never be a Nozzle Check Valve
Terminology:
Check Valve: Check Valves is used synonymously to describe Non-Return Valves as the term Non-Return Valves was used to describe Reflux Valves synonymously two generations ago.
Nozzle Check Valves: Are Co-Axial Check Valves that utilises the Venturi Principal to reduce Headloss and is spring loaded to reduce Waterhammer.
Silent Check Valves: Are Co-Axial Check Valves that are spring loaded for rapid closure but have high Headlosses.
Introduction
A well designed Check Valve is essential in any pressurized pipeline application in order to protect pumps from reverse flow and pipelines from Waterhammer. In addition, the Check Valve should be reasonably priced relevant to the application and have a low headloss characteristic to reduce power consumption.
The specification of Nozzle Check and Silent Check Valves have grown in popularity as part of a Pipeline Surge and Waterhammer protection strategy because of their Non-Slam characteristics. In some cases, the specifications specify either a Silent or Nozzle Check Valve as if though these designs are interchangeable.
Though both valves are Co-Axial designs, they differ substantially in their design approach and therefore their impact on the Life Cycle Costing of a pipeline.
This document briefly discusses the Check Valve Slam phenomenon and the reason why Nozzle and Silent Check Valves are suited for Waterhammer protection. However, it points out that when the complete Life Cycle Costing of the pipeline is taken into account that a Nozzle Check provides a more holistic solution and a lower cost of ownership than a Silent Check Valve in essence, it explains why a Nozzle Check Valve can be a Silent Check Valve but why a Silent Check Valve cannot be Nozzle Check Valve.
Check Valve Slam
The potential for a Check Valve to either create Waterhammer (which is often termed Check Valve Slam) or, prevent Waterhammer is highly dependant on the Check Valve design, the system configuration and the pipeline profile.
Check Valve slam occurs when the valve closing member is slammed suddenly into its seat by flow reversal. Transients created in this manner are damaging to both the valve and the system. Check Valve Slam depends on many interrelated factors in addition to the Check Valve design. The causes or aggravation of this phenomenon can be split into two broad sectors namely:
System Design
Each system design is unique, as the factors that may cause Check Valve slam in one system may be totally different to those that cause slam for the next. Factors to consider are:
Valve Design
Outlined above are factors that can and do lead to high pressure transients upon Check Valve closure. It should be obvious that check valves are not designed equally and therefore react differently to the conditions outlined. Although there is a profusion of Check Valve designs, each with its own design rational, the general consensus from empirical test data conducted over many decades is that a Check Valve must close very rapidly to avoid the destructive phenomenon of Check Valve Slam and all associated pipeline damage.
In order to achieve rapid closure, the travel distance of the closing member (disc travel) of a Check Valve must be short with a low inertia and low friction, and the closing motion should preferably be spring assisted.
Designs such as Nozzle Check and Silent Check valves that have a short stroke and are spring assisted have a better ability to reduce or eliminate Waterhammer than any other designs. This can be demonstrated below.
Figure 1 ' The calculation of the system deceleration is important to determine whether a Check Valve will slam or not. The deceleration is calculated from the original forward flow velocity and time period when the velocity reaches zero. The maximum velocity of the reverse flow depends on the Check Valve construction and on the system deceleration. The higher the deceleration, the higher the reverse velocity that can develop before the Check Valve closes. Therefore, the higher the Waterhammer transient that will be created.
Nozzle Checks provide the fastest response due to its internal design times and therefore possesses the greatest ability to dampen or eliminate Waterhammer, followed by Silent Check Valves. Based on the graph, we can present the different valve designs and the ability to slam from left to right.
Figure 2 ' The distance that the Check Valve closing member has to travel before flow reversal occurs determines the valve's potential to slam. A Swing Check Valve disc has to travel through a 90-degree angle and is often slammed shut by the reverse water flow. A Nozzle Check Valve is a short and linear stroke. In addition, the closure of the valve is spring assisted, closing in that low milli second time span before flow reversal occurs, thereby preventing Waterhammer.
Nozzle Check Valves vs Silent Check Valves
Nozzle Check Valves and Silent Check Valves are both linear closing spring loaded designs with short closing strokes. It is this characteristic that allows the valves to close in a low milli second time span before flow reversal occurs thereby limiting flow reversal and the potential for Waterhammer.
Both Nozzle designs have become more popular in the South African Water Industry since the 's when the awareness of Waterhammer and its damaging effects on pipeline began to grow as a result of greater access to Surge and Waterhammer analysis software and awareness campaigns headed by manufacturers such as REXUS and, academics.
The growing appreciation from end users and design engineers for the need for a Non-Slam Check Valve design is a positive advancement.
However, there is an observation in several specifications where both Nozzle and Silent Check Valves are specified as if they are interchangeable or, where a Nozzle Check Valve is specified but the Valve vendor has supplied a Silent Check Valve instead.
If the selection criteria of the Check Valve is only for quick closure under pump trip conditions, then, either a Nozzle or Silent Check Valve may be suitable. However, if the interest is in the complete Life Cycle Costing of a pipeline, then, there are other criteria to consider such as:
Headloss Characteristics
A Nozzle Check valve is based on a Venturi principle where the internal body profile is specifically dimensioned and contoured to match the performance of a Venturi Nozzle. The disc shape and the flow path is such that the flow velocity increases as the pressure reduces towards the Vena Contracta of the valve. At this point the flow path of the valve increases as the velocity reduces, recovering the head across the valve as the flow exits the valve. This results in a low headloss characteristic.
A Silent Check Valve is designed on the principal that the total valve body area, minus the internal components should be equal or greater than the mating pipe diameter. This design philosophy takes the approach that the less the resistance, the less the losses. However, body internal design configuration is equally important. A design that creates turbulence in the flow will in turn generate head loss.
In the case of the Silent Check valve, because it does not have a diffusor to streamline the flow behind the disc as in the case of the Nozzle Check Valve, therefore creates a higher headloss than a Nozzle Check Valve.
Nozzle Check Valve Silent Check Valve
Figure 3 ' A Nozzle Check Valve is based on a Venturi design that ensure low headloss characteristics. In addition, the relationship between the Diffuser and the profile of the Disc ensures a rapid response to changes in flow. A Silent Check Valve has no Diffuser and therefore creates higher Headlosses than a Nozzle Check Valve
Face to Face Dimensions
Space in a pump-house is generally limited. The shorter the face to face, relevant to the desired performance of a Check Valve, the more compact the design layout can be. TRUFLO manufactures a short face to face Nozzle Check Valve that is in line with API 594 standards and is available in both wafer and flanged versions. This design is on average 40% shorter than a standard Silent Check Valve but with headloss characteristics that are 30% lower than that of a Silent Check Valve.
Initial Cost vs Life Cycle Costing
It is generally understood that the initial capital cost of the Nozzle Check Valve will be higher than that of Silent Check Valve. The differential in price is dependant on the manufacturer. Most Nozzle Check Valve manufacturers, manufacture primarily for the petroleum and gas industry and sometimes venture into the water industry. The cost differential therefore between their Nozzle Check Valves and a Silent Check Valve is high.
REXUS manufacturers primarily for the Water industry and also manufacturers 100% locally. Further, REXUS manufactures both Ring Disc and Centrally Guided Nozzle Check Valves as well as short face to face and long face to face designs. The cost differential therefore between our Short Face to Face Nozzle Check Valves and our Silent Check Valves are negligible specifically if the Life Cycle Cost benefits of the Nozzle Check design is taken into account.
Spot the Difference
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Not all Check Valves are equal. A Silent Check Valve is highly suitable where Waterhammer protection is the prime concern. However, if the design engineer considers the complete Life Cycle Costing of a pipeline system, then a Nozzle Check Valve should in most instances be the preferred choice.
Because a Nozzle Check Valve holds a higher value in terms of capital costs and Life Cycle Costing benefits, manufacturers will at times label their Co-axial Check Valve as a Nozzle Check Valve when in essence, it is a Silent Check Valve. The design engineer can quickly spot the difference by asking the following questions:
How is flow recovered across the valve?
If by a Venturi design, then the valve is likely a Nozzle Check. If by the flow area being equal to or greater than the inlet then, it is likely a Silent Check Valve.
Does the valve have a Diffusor?
If the valve is a Venturi design and has a diffusor then, the valve is definitely a Nozzle Check Valve.
If the design has a diffusor but recovers flow by the area being equal or larger than the inlet then, it is still a Silent Check Valve.
Conclusion
A Check Valve is a critical component on any pipeline project. Both Silent and Nozzle Check Valves are suitable to minimise Waterhammer as they have the fastest response times under pump trip conditions of all check valve designs. In addition, both close silently (hence the name). However, Nozzle Check valves provide a higher performance than Silent Check Valves and are available in shorter face to face dimensions. A quick comparison indicates the differences between the two designs:
Based on the above it is clear that Nozzle Check Valve can be called a Silent Check Valve, but a Silent Check Valve is not necessarily a Nozzle Check Valve. Distinguishing between these two designs makes a big difference when it comes to the Life Cycle Costing of a pipeline system.
A check valve is a device that permits the flow of semisolids, liquids, or gases in only one direction. Check valves, featured in systems ranging from residential plumbing to pharmaceutical production, often rely simply on changes in pressure to manage flows.
In this article, we'll focus on check valves for fire protection, discussing how they work, the types of check valves, and their benefits in fire sprinkler systems. Additionally, we'll help you understand what components you should look for when selecting a check valve and discuss how to maintain them.
Already know what you need? Feel free to go straight to our check valve product page and selection of grooved riser check valve trim kits.
Each fire sprinkler system needs a dependable source of water for immediate use in a fire emergency. Most systems use some combination of gravity tanks, municipal water supplies, and sometimes fire department connections to provide water to sprinkler heads. But while multiple potential water sources provide an extra layer of reliability to fire protection systems, they can also be problematic: drinkable water might be contaminated by water from a fire hydrant, or dirty water from fire sprinkler pipes might flow through nearby streets and into nearby drains. In many other applications, it's simply important that water only goes one way for performance reasons. These include protecting equipment such as pumps and filters from backflow damage, stopping drainage in a fire sprinkler system with elevation changes, providing pressure relief in liquid or pneumatic applications, and more.
Check valves for fire protection prevent the unwanted reverse flow of water, known as backflow, from occurring. Many check valves feature a hinged clapper with a spring, which keeps the valve closed until the water flowing from some water source opens it. These clappers only open in one direction: when these flows reverse (or very nearly stop), the spring presses the clapper against a rubber seat, forming a water-tight seal.
Think of the water on either side of the clapper as the supply side (between the water source and the check valve) and the system side (between the check valve and the rest of the system). When a fire sprinkler head discharges, water is released, decreasing the pressure on the system side. Shortly after, the higher pressures from water stored or pumped from the supply side push open the clapper, and water flows through the system.
When multiple water supplies serve the same fire sprinkler system, each must have a check valve.
From the Edition of NFPA 13
16.9.4.1 Where there is more than one source of water supply, a check valve shall be installed in each connection.
Check valves decide which source actually provides the system with a supply of water at a given moment. When multiple water sources are used simultaneously, only the check valve connected to the highest-pressure water source remains open. Imagine you have a system with three water sources at the following pressure ratings, expressed in pounds per square inch (PSI):
Active Water Sources Check Valve Status 100 PSI Fire Department Connection (FDC)(Note: these pressure levels are provided only as illustrations and are not meant to be taken as typical for these water sources.)
When water is supplied from all three sources, only the check valve leading to the fire department connection remains open. Why?
Of these three sources, the FDC delivers the highest water pressure. This supply pressurizes the water on the system side. This increased system-side pressure is greater than the 75 or 50 PSI on the supply side of the check valves connected to the other water sources and, as a result, their clappers close.
To continue with this example, if only the city water and the gravity tank supply water, the check valve to the 75-PSI gravity tank will open, while the city water's check valve stays closed. Only when all other sources are exhausted or closed does the municipal water supply open the check valve and flow into the system.
There are four main types of check valves for fire protection:
An alarm check valve consists of a spring, a valve clapper, and an alarm port. The alarm port can be used to direct water to a water motor alarm or an alarm pressure switch used to alert nearby personnel to the system's activation or send a signal to a fire alarm control panel.
A swing check valve can act as an alarm check valve for a wet-pipe fire sprinkler system, but only when that valve can sound an alarm.
From the edition of NFPA 13
16.11.3 Waterflow Detection Devices.
16.11.3.1 Wet Pipe Systems.
The alarm apparatus for a wet pipe system shall consist of a listed alarm check valve or other listed waterflow detection alarm device with the necessary attachments required to give an alarm.
Typically, these alarms are sounded by flow switches, which send a signal to local alarms and/or the fire alarm control panel when water flows through system piping.
Grooved check valves are a type of swing check valve for use in systems that have grooved-connecting pipe ends. Many grooved check valves can be used in a vertical or horizontal orientation, allowing them to be used in various configurations with a fire department connection, bypass connection, gravity pressure tank, or pump discharge connection.
A grooved shotgun riser check valve is similar to a grooved check valve but includes pressure gauges to monitor the supply and system pressure. The shotgun configuration, which uses an electric water flow switch and an alarm bell, is only permitted in situations where a water motor alarm is not required. Shotgun riser check valves offer a more compact and economical alternative to an alarm check valve.
Finally, there are backflow preventers that are explicitly designed to prevent the contamination of water supplies. They use check valves and extra fail-safes to make absolutely sure water flows one way and doesn't contaminate potable water supplies. You can read more about them in our other blog: 'Backflow Preventer vs. Check Valve: What's the Difference?'
Yes, check valves require periodic inspections and maintenance. For this reason, NFPA 13 warns against burying check valves or positioning them in areas where they will be inaccessible.
From the Edition of NFPA 25
13.4.2 Check Valves
13.4.2.1 Inspection. Valves shall be inspected internally every 5 years to verify that all of the valve's components operate correctly.
13.4.2.2 Maintenance. Internal components shall be cleaned, repaired, or replaced as necessary in accordance with the manufacturer's instructions.
Alarm check valves and system riser check valves require more frequent inspections. Regular inspections help to verify that the clapper functions properly, that the valve seat is in good condition, and that there aren't any visible signs of corrosion or blockages.
From the edition of NFPA 25
13.4.1.1* Alarm valves and system riser check valves shall be externally inspected quarterly and shall verify the following:
(1) The gauges indicate normal supply water pressure is being maintained.
(2) The valves and trim are free of physical damage.
(3) All valves are in the appropriate open or closed position.
(4) The retarding chamber or alarm drains are not leaking.
(5) Where applicable, the valve in the connection to the pressure-type contacts or water motor'operated alarm devices are either sealed, locked, or electrically supervised in the open position.
NFPA 25 also requires an inspection of alarm valves' internal components every five years.
13.4.1.2* Alarm valves and their associated strainers, filters, and restriction orifices shall be inspected internally every 5 years unless tests indicate a greater frequency is necessary.
Maintenance should be performed by following the guidelines set by the manufacturer. Following an inspection, the manufacturer's guidelines should also be used while returning the system to service.
Every check valve installed on a fire protection system should be FM-approved and/or UL-listed. Listings certify that a check valve is designed to withstand the conditions it'll be exposed to while in use. They're also specifically required by NFPA for check valves on fire sprinkler FDCs and the alarm check valves used with wet-pipe fire sprinkler systems.
From the edition of NFPA 13
16.11.3 Waterflow Detection Devices.
16.11.3.1 Wet Pipe Systems.
The alarm apparatus for a wet pipe system shall consist of a listed alarm check valve or other listed waterflow detection alarm device with the necessary attachments required to give an alarm.
16.12.6 Valves.
16.12.6.1 A listed check valve shall be installed in each fire department connection and shall be located in an accessible location.
In addition, when you're purchasing a check valve for fire protection, you should also consider:
QRFS carries UL-listed and FM-approved grooved check valves for pipes ranging from 2 to 6 inches. All of our check valves feature a non-slamming, spring-loaded clapper that forms a water-tight seal against an EPDM synthetic rubber seat. With a durable ductile iron exterior and corrosion-resistant stainless steel clapper, these valves are built to last.
Any of these valves can be installed in a vertical or horizontal orientation. What's more, each can be used as an alarm check valve with a shotgun riser check valve trim kit, available for every check valve we carry.
When you make a purchase from QRFS, you receive quality products at our competitive pricing. We begin processing your order the moment we receive it so we can provide you with the fastest standard delivery possible, generally reaching you in 2-3 business days.
View our selection of grooved, inline, and swing check valves, plus our inventory of grooved riser check valve trim kits.
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This blog was originally posted by Jason Hugo and Anna Hartenbach at blog.qrfs.com on October 26, , and updated on January 1, . If this article helped you select the right check valve, check us out at Facebook.com/QuickResponseFireSupply or on Twitter @QuickResponseFS.