Fire protection pumps: Updates to NFPA 20

06/25/2013


Pressure relief valve  

Issues with pressure relief valves continue to surface, with one committee member reporting a significant number of events where a pressure relief valve that was piped back to the fire pump suction would apparently be completely open. This results in a significant impairment to the fire protection system that may go unnoticed until the water is needed for an actual fire. This issue can occur any time the discharge from a pressure relief valve cannot be observed. When a pressure relief valve is piped back to a suction tank, a means to observe the discharge should be provided (see the sight glass in Figure 3, item 2).

Figure 3: Pressure relief valve and flow meter piped to suction. Courtesy: Aon Fire Protection Engineering

A fire pump adds energy to the water passing through it. When no water is discharging, the energy will be in the form of heat added to the water. To prevent water overheating at churn, a circulation relief valve located between the fire pump and fire pump discharge check valve is required to operate at churn pressures, but should not operate when the fire pump is flowing water (see circulation relief valve in Figure 4). When water is flowing through a pressure relief valve or a test loop piped back to suction, the pressure will be lower than churn pressure and the circulation relief valve designed to operate at churn will not operate. However, when circulating water though a closed loop, a fire pump adds even more energy to the water passing through it than under churn conditions. An additional circulation relief valve is required when a relief valve is piped back to the pump suction. This additional circulation relief valve should be set to operate below the 150% pump discharge pressure.

Figure 4: A fire pump circulation valve is shown in this photograph. Courtesy: Aon Fire Protection EngineeringWithout proper operation of the circulation relief valves, the temperature of the water will rise. In addition, if the pressure relief valve is wide open, it may cause a diesel engine (if used) to exceed its rated horsepower. When the higher temperature water is sent to the heat exchanger for cooling, the engine may overheat. In one case I am aware of, a diesel engine was destroyed and this scenario appears to be the cause.

An argument was presented that if the pressure relief valve discharged to the atmosphere, the extreme flow of water would not be ignored. Currently pressure relief valves are permitted and required only to prevent overpressurization for two reasons:

  1. Diesel overspeed conditions
  2. Failure of the variable speed mode on variable speed pumps.

Under normal operating conditions the pressure relief valve should not operate even under churn conditions. However, older fire pump installations may use the pressure relief valve to “trim” the fire pump discharge and the pressure relief valve may discharge a large volume of water well beyond churn conditions. The committee felt that the pump installations in the reported incidences did not conform to current requirements and returning pressure relief valve pipe to the pump suction was appropriate for the limited use of pressure relief valves permitted by NFPA 20.

While piping a pressure relief valve or test loop back to suction is still permitted, a requirement was added for heat exchanger cooled diesel engines to monitor the temperature of the cooling water and shut 4down the engine while operating in the test mode (only) whenever the water temperature at the engine inlet of the heat exchanger exceeds 104 F.  

As noted earlier, a separate circulation relief valve is required by NPFA 20 whenever the pressure relief valve is piped back to suction. The additional circulation relief valve is critical to prevent the fire pump from overheating. Tier 3 environmental requirements that limit the intake air temperature cannot be maintained without limiting the cooling water temperature. A circulation relief valve on a pressure relief valve piped back to suction is shown in Figure 3, item 9.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.