Owner's Manual

Eliminate Water Hammer

WHAT IS WATER HAMMER?

Water hammer (or hydraulic shock) is the momentary increase in pressure inside a pipe caused by a sudden change of direction or velocity of the liquid in the pipe. Water hammer can be particularly dangerous because the increase in pressure can be severe enough to rupture a pipe or cause damage to equipment. 

CAUSES

It’s a simple fact that liquid flowing in a pipe contains two types of energy: potential energy and kinetic energy. Kinetic energy is attributed to its velocity and potential energy is represented by its pressure. Neglecting friction, the combination of kinetic and potential energy remains constant at all points throughout the length of the pipe. Changing the kinetic energy by changing the liquid velocity forces the pressure in the pipe to change. If the velocity of the liquid decreases (decrease in kinetic energy), the liquid pressure increases (increase in potential energy). 

Water hammer most commonly occurs when a valve is closed quickly and suddenly stops the flow of liquid in a pipeline. When this happens, shock waves travel back and forth through the piping system equal to the speed of sound in that liquid (for water at 70 degrees that is over 4800 ft/sec). These waves travel backward until encountering the next solid obstacle (frequently a pump or check valve), then forward, then back again until pressure is equalized.
Additionally, the sudden closure of a valve in a pipeline causes the momentum of the liquid column to exert a force on the valve’s shut-off element (disc, gate, or ball). This sudden separation of the water column causes two things to happen simultaneously: the pressure on the upstream side of the valve increases and pressure on the downstream side of the valve decreases. The liquid downstream of the valve will attempt to continue flowing, creating a vacuum that may cause the pipe to collapse or implode. This problem can be more serious if the pipe is on a downhill slope. 

To prevent a sudden change of pressure near the valve’s shut-off element, air and vacuum relief valves, or air vents, are installed just downstream of the valve to allow air to enter the line and prevent a vacuum from occurring.

Therefore, the proper opening and closing of valves is fundamental to safe pipeline operation. Closing a valve at the downstream end of a pipeline creates a pressure wave that moves in the upstream direction. Closing a valve in less time than it takes for the shock wave to travel to the end of the pipeline and back is called “sudden valve closure”. Sudden valve closure will change velocity quickly and can result in a pressure surge. 

OTHER CAUSES OF WATER HAMMER

•   Rapid pump startup can induce the rapid collapse of a void space that exists downstream.
•   Rapid pump shutdown can create a quick change in flow, which causes a pressure upsurge on the suction side and a pressure downsurge on the discharge side. Of the two, the downsurge is usually the major problem. The pressure on the discharge side reaches vapor pressure, resulting in vapor column separation.
•   Check valve slam (Due to sudden deceleration, a check valve may slam shut rapidly.) 
•   Movement of air pockets in a pipe. Air is compressible and if carried along in a pipeline, can act like a spring, being compressed at low spots in a line and expanding at high spots in the line. Compression and expansion produces pressure variations which, if great enough, could produce serious water hammer pressures. 
•   Water-column separation can also result in serious water hammer pressure values when the separated column rejoins at high velocity. 

CONTROL

The following steps can be taken to reduce or eliminate water hammer:

  •  Proper education and training of personnel on the dangers of water hammer and how to mitigate them through proper opening and closing of valves.
  •  Use start-up and shut-down procedures for pumps that reduce the possibility of creating water hammer conditions.    
  • Reduce the velocity of the liquid in the pipe. To keep water hammer low, some references recommend keeping the flow velocity at or below 5 ft./s.  
  • Use slow-closing valves. Anything with a wheel, like a gate valve, is generally considered slow-closing. Valves with handles, like butterfly valves or ball valves, are considered fast-closing.  
  • Use pipe with a higher-pressure rating. For example, DR 26 HDPE pipe is rated for 65 psi, whereas DR 11 HDPE is rated for 160 psi.   
  • Air valves are often used to remediate low pressures at high points in the pipeline by admitting air into the line to reduce the possibility of partial vacuum and possible pipe collapse.   
  • Install pressure relief valves to prevent excessive pressure in the pipe.  
  • Use air chambers, surge vessels, accumulators or expansion tanks that are partially filled with air or gas and cushion possible shock.

 


 

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"Root Cause" Accident Investigation - Not a Band-Aid, but a Solution

In route to repair a water main leak, a newly hired backhoe operator drifts to the edge of the road and slams into the back of a car, injuring a mother waiting for her kids to get out of school. 

The young operator is near tears, the supervisor overflows with accusations, another worker slams their hand in a door. Everything is confusion.

Eventually, when the mess is sorted out, the safety coordinator will investigate the accident. He learns the steering and brakes on the backhoe were bad. Someone will get blamed, and the equipment issues will be repaired.  The safety coordinator, overwhelmed by the demands on his time, will go off to fight the next fire. 

This is not an extreme case.  Accidents are handled like this every day. At best, this type of approach deals with symptoms and not the actual or root cause.  In a few days, another piece of equipment, perhaps a manifold at the water plant, will fail.  Someone else will be injured or maybe killed.  The plant will shut down for a while and the damage will be repaired, but the risks will remain.

To identify and control risk, an accident investigation must get to root causes. Why was a new employee operating the backhoe?  How much training had they received?  Why wasn’t the faulty equipment taken out of service immediately?  Why wasn’t it clearly tagged out of service? Reported? Was the equipment regularly inspected?  Is there a preventative maintenance program? What must be changed in maintenance, training or safety to keep this from happening again?

Accident investigation should be a critical part of overall safety program strategy. Done correctly, it can enhance safety and reduce costs.  All accident investigations should be conducted in a professional manner and should always focus on causes: the why's. Using the 5 “Why’s” of a typical Root Cause Analysis allows the employer to discover the underlying or systemic, rather than the generalized or immediate, causes of an accident.  Correcting only immediate cause may eliminate a symptom of the problem, but not the problem itself. The more incidents that are reported, the more problems can be investigated and resolved. The more problems solved, the safer and more cost effective the operation will be.

The fact is that the only difference between a near miss and a catastrophe may be chance. That's why every potential problem should be resolved. 

For more information on conducting accident investigations, and developing a Root Cause Analysis please view “Incident [Accident] Investigations: A Guide for Employers” https://www.osha.gov/dte/IncInvGuide4Empl_Dec2015.pdf

Accidents and injuries are not a cost of doing business; all are preventable!

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Hearing Protection - "What Did You Say?"

“Blindness separates us from things but deafness separates us from people.” – Helen Keller

Human beings are social creatures: we drive in packs on the highway, we constantly use social media and we enjoy family and friend get-togethers.  A major part of our ability to socialize involves our ability to hear and communicate.  We all have someone in our family, or know of someone, that has difficulty hearing or diagnosed hearing loss.  We start a conversation with them but soon tire of repeating ourselves and eventually we avoid speaking to them altogether.

Approximately 15% of Americans between the ages of 20 and 69, or 26 million Americans, have hearing loss that may have been caused by exposure to noise at work or in leisure activities. The best prevention approaches for high noise levels are: remove the noise, remove the worker and then protect the worker by using Hearing Protection Devices (HPD). HPD are a type of Personal Protective Equipment (PPE) worn to reduce (not eliminate) the level of sound entering the ear. PPE is the last line of prevention for a hazard; however, for most situations, HPD are the main defense against Noise Induced Hearing Loss (NIHL).

When should you wear HPD:
    If the noise is measured at 85dB and above
    If you must raise your voice for a person 2-3 feet away to hear you
    If, after leaving the noise area, sounds are “dull, flat or muffled”
    If your ears “ring” after exposure to noise

It’s a good idea to wear HPD with these types of equipment:
    Lawn Mowers and Weedeaters
    Gas Powered Demolition Saws
    Pneumatic Impact Tools

Noise Reduction Ratings (NRR) are required to be printed on all HPD, to aid the wearer in selecting the correct protection for a situation.  The NRR value provides information on the measured reduction of noise in a laboratory setting.  In real world protection, NRR values are overestimated; as such, the National Institute of Occupational Safety and Health (NIOSH) recommends derating HPDs to provide the user a more accurate NRR value in real world situations. NIOSH suggests derating ear muffs by 25%, foam plugs by 50% and molded flanged plugs by 70%.  For example, if you have a pair of foam ear plugs with a NRR=30dB, derating by 50% results in an estimated noise reduction of 15dB.  It should be noted that derating is still only a rough guide and actual protection can vary.

Noise Induced Hearing Loss can be prevented by using the correct hearing protection for your situation and wearing it properly. Failing to use HPD or using it incorrectly, even during short exposures, can have negative consequences; and once noise-induced hearing loss occurs, it cannot be cured or reversed.

The decisions we make today will affect our quality of life on down the line.  It may be easy to convince yourself that everything is okay: “I will only be using the equipment for an hour,” “My ears only ring for a little while after work,” and “It’s part of the job.”  This kind of shortcut is a bad idea in the long run.  Make the right choice and wear your hearing protection.

For more information on Noise Induced Hearing Loss or Proper Use of Hearing Protection Devices please check out OSHA.gov, use the OSHA quick-card at https://www.osha.gov/Publications/3498noise-in-construction-pocket-guide.pdf

Stay tuned to OMAG’s YouTube channel for a video on Hearing Protection soon.

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Caring for Oklahoma Municipal Cemeteries (Part 1)

As a service to our members, OMAG’s Risk Management Services Department has developed a series of articles addressing various challenges and issues municipal cemetery caretakers might face.  Although tort claims generated from cemeteries aren’t at the top of the frequency or severity list, these types of claims don’t bode well for our public image and are most certainly preventable.

To our citizens, or customers, a cemetery serves two purposes, (1) the burial of loved ones and (2) a place where the living memorialize the dead.  Cemeteries are scattered across the landscape and are found in almost all communities. In addition, many of our cemeteries predate statehood.  Cemeteries and their symbols guide us into the past. Markers tell us not only about the individual who died, but the marker’s appearance and placement provides information about the society from which the individual departed. The symbols and text inscribed on markers contain important information for understanding the past. Consequently, cemeteries and markers are studied by art historians, folklorists, cultural geographers, archaeologists, anthropologists, genealogists, and historians. When cemeteries are ignored and allowed to deteriorate, and markers are damaged or destroyed, society loses important information about the past. Ultimately, an important part of ourselves is lost.

Maintenance and preservation of our cemeteries go hand in hand and are performed out of respect for those who are buried. Maintenance is simple, but may not be easy.  Keeping the grass mowed, trees trimmed, and good lighting not only enhances the overall look of the cemetery to the general public but also discourages vandalism.  Proper use of tools to keep cemeteries free of grass and limbs must be observed diligently in cemetery maintenance. Power mowers and weed eaters, when carelessly used, have and can still scar and break tombstones, especially old ones since they are usually made of softer stone. So, to minimize damage to old stones, grass near these stones should be pulled by hand. When power mowers and weed eaters are used guards should be in place to deflect any debris that is being thrown that may cause damage to the stones. When mowing, it is best to use a mulching mower when possible since there is not an outward discarding of debris. Any cracked or broken stone should be addressed immediately so to minimize further damage to the stone.

Lighting is also essential to maintain security in a cemetery. Lights should be checked regularly
and changed if needed. Good lighting reduces the chance of vandalism. It is cheaper to change a
bulb than repair vandalized stones. A well-lit cemetery draws the public to keep an eye on the
cemetery which deters vandalism. A regular check of the cemetery is also essential. Look for stones that have cracked, been damaged or destroyed, and address the defects as soon as possible.  Also, check fences for needed repairs and vandalism.

Day-to-day, periodic, and long-term maintenance are essential components of a long-term plan to ensure the continued preservation of any cemetery. The establishment of a continuous maintenance program is the most effective preservation activity that can be performed by a municipality. A well-kept cemetery will discourage unwanted behavior within your site. A plan that includes maintenance, landscaping, training, and funding of a permanent work force will help preserve your cemetery.  

Look for future articles, publications, and training events that will assist you in maintaining and managing your municipal cemetery.

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Seven Ways to Increase Safety on a Budget

Oklahoma’s staggering economy is causing decision makers in every municipality to look for ways to stretch budget dollars. This means that the safety of your workplace is becoming more important than ever. Workplace injuries are becoming more expensive and in turn affect the “bottom line” of every municipal government.

Sometimes safety isn’t foremost in our minds. For some, a severe injury to an employee is a remote possibility and hardly worth worrying about. For others, the risk of injury seems out of proportion to the financial rewards that can be gained. The justification of this reasoning is “if something happens, insurance will cover it.” Still others believe that there is no way to get the job done safely without spending money they just don’t have.

The belief that municipal decision makers must choose between working safely and maintaining an already strained budget is a wrong paradigm.  Here are seven things decision makers can do to reduce the risk of worker injuries without adding prohibitive cost:

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Case Study: Death in Sewer Manhole

The following article is taken from the Occupational Safety and Health Administration’s Fatal Facts newsletter. It proves that unauthorized entry into sewer manholes is always extremely dangerous and sometimes fatal.

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