Boiler Blog | Nationwide Boiler Inc.

The safety valve is one of the most important safety mechanisms in a steam system. Not only are they required by code, but most importantly, safety valves provide a measure of safety for plant operators and for the equipment.

The American Society of Mechanical Engineers (ASME) governs the code that establishes the requirements for safety valves, therefore it key that all plant personnel are familiar with current codes that apply to their system.

We found the following sizing guidelines and installation tips listed in Process Heating Magazine and thought the information would be useful to pass on. We hope that this enhances your knowledge and understanding of safety valves.

Sizing Guidelines

The two major considerations for safety valves are proper sizing and correct installation. The following tips address safety valve sizing.

• It is suggested that the setpoint selected for the safety valve provide a differential of at least 20 percent between operating and set steam pressures.


• When considering installation of a safety valve downstream of a steam pressure control valve, the total capacity of the safety valve at the setpoint must exceed the steam control valve's maximum flow capacity (the largest orifice available from that manufacturer) if the steam valve were to fail to open. The inlet steam pressure to the valve must be calculated at the maximum safety valve setting of the steam supply source, not the nominal operating pressure.


• It is important not to oversize a safety valve. Bigger is not better in this case because a larger-than-required valve could cause chatter, leakage and premature failure.


• Many times, a single safety valve is not possible due to high capacity, physical limitations or economic considerations. An acceptable alternative is to employ multiple safety valves on the same system. The valves should be of the same setpoint and the capacities must be equal to or greater than the rating of the equipment. Additionally, the vent pipe must be sized to account for the venting capacity of all of the safety valves fully opening at the same time.


• The set pressure of the safety valve should be set at or below the maximum allowable working pressure (MAWP) of the component with the lowest setpoint in the system. This includes steam boilers, pressure vessels and equipment, and piping systems. In other words, if two components on the same system are rated at different pressures, the safety device protecting both of these devices must be set at the lower of the two ratings.

Installation

Once sizing has been properly determined, proper installation is the next crucial step to ensure safety. There are several points to consider when installing the safety valve.

• The steam system must be clean and free of any dirt or sediment before commissioning the steam system with a safety valve.


• The safety valve must be mounted vertically with the valve's spindle in the vertical position.


• The inlet steam piping to the safety valve must be equal to or larger than the safety valve inlet connection.


• There should be no intervening shutoff valves located between the safety valve inlet and the steam component that could permit the safety valve to be isolated from the system.


• Drains or vent openings on the safety valve should not be plugged or capped. They are on the safety valve for a reason.


• Safety valves are set, sealed and certified to prevent tampering. If the wire seal is broken, the valve is unsafe and should not be used. Contact the supplier immediately.


• For multiple safety valve installations using a single connection, the internal cross-sectional area of the inlet shall be equal to the combined inlet areas of all the safety valves.


• All safety valves should use a drip pan elbow on the outlet. The drip pan elbow changes the outlet of the safety device from horizontal to vertical. Installation of the drip pan elbow has its own guidelines, which should be researched and addressed to meet the needs of each application.


• Never attach the vent discharge piping directly to the safety valve. This would place undue stress and weight on the valve body. Also, the safety valve vent pipe may not touch the drip pan elbow.


• The drains on the drip pan elbows are to direct condensed vapor and rain safely away to the drain. Do not plug these openings.


• Steam will not escape from the drip pan elbow if the vent line is sized correctly.

Vent Piping

There also are some important considerations when it comes to the vent piping of the safety valve and the steam system.

• The diameter of the vent pipe must be equal to or greater than the safety valve outlet.


• The vent line should be sized so that back pressure is not placed on the drip pan elbow.


• The length of the vent pipe should be minimized where possible.


• The discharge outlet of the vent pipe should be piped to the closest location where free discharge of the safety device will not pose a safety hazard to personnel. For a roof-line termination, the vent should be no less than 7' above roof line. The top of the vent line should be cut at a 45° angle to dissipate the discharge thrust of the steam, prevent capping of the pipe and to visually signify that it is a safety valve vent line.

Unscheduled plant outages, no matter what the cause, result in increased costs and loss revenue. As plants continue to reduce personnel and maintenance budgets, the likeliness of an equipment failure is more and more common. Below are just a few guidelines for effectively managing your plant equipment, while utilizing limited resources:

Know your equipment's age.

Replacing or upgrading obsolete, aging or high maintenance equipment before it fails can prevent serious failures in the future. Obtain and file a record of all your major pieces of equipment. Note when the equipment was acquired, built and record any maintenance items performed. You may also want to take pictures of your equipment, including any serial numbers and nameplates. In addition, be on the lookout for new equipment models that have been re-designed with technological upgrades that may result in immediate cost savings (decreases in energy consumptions or increases in efficiencies). You may received an immediate cost savings or rebate.

Continually train your operators and plant personnel.

Human error is bound to happen, but these errors can be drastically reduced by continual training. Training comes in all forms and it does not necessarily have to be formal, classroom type lectures. Utilizing check-off lists for routine tasks is an easy and low cost method to ensure that all critical task are carried out, no matter how small. Also, promote common maintenance reminders through the use of signage.

Develop a schedule.

Scheduling major inspections and repairs at the same time helps to minimize unscheduled outages and makes the most of a schedule.  Take advantage of that time to conduct anything else that needs repaired, inspected or replaced. Overall, having a plan in place makes it easy to be proactive in your planning and scheduling of people and resources.

Do you have additional guidelines you would like to share? Let us know.

All boilers are subject to damage if proper chemical treatment, water analysis and blowdown procedures are not followed. If these items are ignored, both during operation and idle periods, serious damage to the boiler will result.

When Nationwide Boiler delivers a rental unit, it is the boiler operator's responsibility to maintain and supervise the water treatment conditions of their rental boiler in order to avoid any additional charges that may result from improper operation or maintenance. It is recommended that the services of a reputable boiler chemical consultant are retained in-order to supervise the water treatment conditions on a regular basis. At least once each week, daily boiler logs, chemical treatment tests, a chemical treatment consultant's report and maintenance records should be implemented and sent to Nationwide Boiler. This ensures that the proper maintenance is being done on the equipment and provides accurate maintenance reports for Nationwide's equipment files.

As a boiler operator it is important to inspect the waterside condition of watertube boilers that may result in heavy scale or oxygen pitting or corrosion. Waterside scale can be cause for concern on the reliability of a boiler by the following:

• If the boiler tubes contain heavy scale, tubes can overheat and/or fail leading to tube leaks


• If the boiler has some scale but it is not heavily deposited, certain types of water treatment plans can cause the scale to quickly be removed in large flakes, plugging tubes and eventually leading to overheating and tube failures


• Certain types of scale can be easily removed such as a conventional soft phosphate scale, while others are very difficult to remove such as silica

If scale is present, any scale greater than 1/8 inch or 0.125 inch should be removed from the boiler prior to placing it back in operation.

Oxygen corrosion is not as easily detected as scale. O2 pitting can occur under deposits and in very small areas that can escape detection by the naked eye. If O2 pitting is suspected, a tube needs to be removed from the boiler for more close examination.

Therefore, it is extremely important to assess the waterside condition of a boiler and then to determine the type of scale and the method for removing it. There are several methods for removing boiler scale including high pressure water blasting and other proprietary methods offered by water treatment representatives. Overall, with proper boiler maintenance and water treatment plans, such procedures can be avoided.

Examples of (1) Heavy Scale, (2) Medium Scale and (3) No Scale

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The proper removal of rental boiler equipment is a critical step when renting boilers and it ensures the safe return of equipment to the rental company's facility. It also reduces the risk of additional costs to the customer. By taking a few precautionary steps as described below, the boiler can be properly prepared for pick-up and shipment.

Drain & Flush

First, completely drain and flush all the boiler lines with clean water and open all water drain valves. This will minimize the chance of any corrosion and hard caused by unforeseen freezing conditions.

Inspect

Second, Inspect all fireside and waterside surfaces and report any visual damage, scale build-up, or refractory problems. Having photographic evidence of the boiler condition after the rental period is advisable.

Stack Removal

Next, the stack should be removed, remounted and Bolt down stub stack, non-return valve, safety valves and vent valve to trailer (same location as it was received).

• Cover all boiler openings (stack, steam, gas, water, electrical panel, safety valve openings)


• If trailer is disassembled, reattach wheels and gooseneck


• Check tire pressure


• Return all instruction and operating manuals to the rental company via the trucker or transportation company responsible for returning the unit

If you have questions about preparing your unit for pick-up, don't hesitate to call Nationwide Boiler at 800-227-1966.