I have seen several situations where there was an attempt to meet a code requirement and yet all requirements were not satisfied. The best way to explain this is to provide an example. A new nuclear facility is adding a storage tank of combustible liquids in the turbine building. One of the concerns addressed is the containment or drainage of the liquids. The typical requirement is to contain the contents of the tank (or largest tank) plus the flow of fire protection water for a specified period of time. This containment could be by several methods including a dike arrangement. The designer could impose the requirements of the nuclear insurer which in most cases would be a 10 minute requirement. This would be correct for compliance to the nuclear insurer’s requirements. However the NRC, via NFPA 805, would require a 30 minute flow requirement. And to make this even more fun the International Building Code through a reference to the International Fire Code results in a 20 minute requirement. So which one do you implement? The correct answer is (1) you determine which codes and standards are required by local, state, insurer and government (NRC) agencies. (2) You apply the most restrictive of the applicable standards. See…pretty simple ; ).

Trust and personal integrity is important in the fire protection discipline as it is in every engineering discipline. However in fire protection it has special significance since fire protection crosses over to many other disciplines. Fire protection requires significant judgment along with the correct interpretation of codes and standards. It is not uncommon to make interpretations of the codes to suit ones purpose and achieve the end goal if there is a sound basis for these interpretations. However I have seen interpretations that were completely inappropriate with the end goal of reaching the desired answer with no regard to how the information was manipulated to reach the answer. For example an analysis regarding the failure of structural steel credited a suspended ceiling as a fire barrier thus preventing the fire exposure to the building steel located above the suspended ceiling. The desired answer was that structural steel fire proofing was not required. These suspended ceilings systems typically consist of 2’ x 4’ ceiling tiles held in place with light perimeter strips and runners spaced such that the ceiling tiles can be positioned in place. Structurally this is very light and provides only a negligible fire barrier for the structural steel located above the ceiling tiles. (There are fire rated arrangements which detail the thickness and location of support wires, etc, but this was not the case for this situation). In another situation in order to avoid the protection of lateral hose station piping (equivalent to the rating of protected stairways) the convenient interpretation was made that the water filled fire protection piping was a fire barrier and thus the code requirement was satisfied. These are good examples where the desired result caused interpretations that were without a basis and raised the issue of credibility. It is important to note that we are obligated (professionally and morally) to provide the most correct answer which often is not the desired answer.

In a previous post I discussed pump sizing and provided an example of how not to size a fire pump. Internet searches provide all kinds of information on sizing a pump but most of these assume you know exactly what the demand, both pressure and flow, will be. Unfortunately this is not how it is in the real world. In addition this blog does not have enough room (and the author will quickly become tired of all the typing) to go into every detail but perhaps these pointers will help. One of the initial items to consider is the water demand. You can estimate this demand based on similar past installations and educated estimates. For example the systems that are a candidate for the highest demand are either those in the turbine building or the transformers. For turbine building systems the required demand is typically on the order of .3 gpm per sq. ft. for an area of 5000 sq. ft. Thus the flow can be estimated as 1500 gallons per minute. Add 10 to 20% for system imbalance and then add 500 gpm for hose streams and the total flow will be about 2300 gpm. A similar but slightly more complicated flow demand can be estimated for the transformers (or use a value from previous transformers provided at another facility). So now we have an estimate of the flow requirements. Now you need to consider the pressure drop in the supply piping from the pump location to the flow location. You have to consider the hydraulically shortest route out of service (some use single longest route), hydraulic characteristics of the pipe at the end of plant life (C factor), elevation, and other users that may be in service at the same time (which are not supposed to be allowed but seem to creep in many plants licensing basis). This produces an estimate of the pressure demand in the system due to elevation and pressure drop. Then an educated estimate (such as a review of previous sprinkler system demand requirements from other sites) should be made of the pressure available for a suppression system of the size anticipated. This value along with the pressure drop in the supply piping and elevation will provide the estimated required pressure of the fire pump. So let’s say the numbers produced are 2500 gpm and 110 psig (note that if the water supply is a storage tank the head of the tank is not credited). NFPA 20 recommends that a pump should be selected based on an operating range of 90% to 150% of its rated capacity. Pumps have defined flow capacities so for this flow demand we would select either a 2000, 2500 or 3000 gpm pump. I may select 2500 gpm. Someone else may select 2000 gpm. So it would be possible to select a pump rated at 2000 or 2500 gpm at 110 psig. I would select a standard size such as 2500 gpm at 125 psig. Other factors that must be considered include cost, potential impact on the use of pressure reducers at hose stations, pressure rating of system piping and fittings, pump shutoff/churn pressure, jockey pump pressures, desired curve, set points and numerous other factors. There are variations of the selection process dependent on the site circumstances but this should at least get you away from the incorrect method described in an earlier post. Perhaps the best path is to estimate your anticipated flow and pressures and then seek the advice of a fire pump vendor.

It is a common misconception that the “Class” designation means the component is good for that pressure rating (150 psig). For example NFPA 13 includes a reference to ASME B16.5 for steel pipe flanges and flanged fittings. It includes several Classes including Class 150 and Class 300. The Class rating is derived from a temperature pressure relationship and represents the maximum allowable working gage pressure. There are tables in ASME B16.5 which reveal that up to 100F the allowable working pressure is 285 psig for Class 150 (using A105 which is the most common forged material of group 1.1 under ASME/ANSI B16.5 ). At a temperature of 200F the allowable working pressure is 260 psig. The code allows interpolation between temperatures within a class but does not allow such between class designations. So the bottom line is that the 150 is a dimensionless designated number and the allowable working pressures is much higher considering the typical temperatures of fire protection systems.

The typical fire pump and suction tank arrangement has the tank and pump in close proximity to each other such that the suction lines have relatively short equivalent length. In addition for commercial nuclear applications the typical arrangement consists of two fire pumps and tanks with a cross connect that allows a pump to take suction from either tank. NFPA 20 contains a requirement that the suction pipe size be such that with all pumps operating at 150% the gauge pressure at the pump suction is 0 psi or higher. In addition the code contains an exception where the suction tank base is at or above the elevation of the pump the gauge pressure at the pump suction is permitted to drop to -3 psi. Normally this is not an issue however some new plant designs (and perhaps some existing plants) are arranged with the pumps and tanks separated (having a long cross connect line) by a large amount of equivalent pipe which could impact the 0psi/-3psi requirements. Many engineers perform suction calculations to verify that the velocity does not exceed 15 ft/sec but ignore the suction gauge pressure requirement because it typically is not an issue. Caution should be exercised and these calculations should always be performed as it is quite possible under some physical arrangements to not meet the code/performance requirements.

I have seen several installations of HDPE pipe where the engineer only considered the hydraulic properties of the plastic pipe.  For example the use of HDPE pipe to bypass a section of severely deteriorated unlined carbon steel pipe.  The C factor of plastic pipe is far superior to that of metallic pipe and its C factor does not degrade over time.  So why not use HDPE?   A factor that is often not considered is design life (most nuclear plants are in the design extension stage of their original 30 year life and the new plants are being licensed for 60 years).   Using PE100 plastic pipe as an example shows that the expected design life at 68 degrees F is about 100 years.  Increase the temperature to 104F and the life is about 50 years.   Further increase the temperature to 140F and the design life is about 25 years.  Even more impacting is the service pressure of the pipe.  Class 150  and Class 200 pipe are rated at 73.4 degrees F.  At 100F the pressure is 121 psig and 162 psig respectively.  Temperatures of 140F produces a pressure rating of 80 psig for Class 150 and 100 psig for Class 200 pipe.   So….. is your ground temperature 73.4 degrees F?  Are your above ground installations subject to maximum temperatures of 73.4 degrees F?   Is your water supply (including jockey pump and fire pump shutoff pressure) higher than your pipe pressure rating at the actual temperature?

Fire dampers require a space around the damper for thermal expansion. This is stated in the damper manufacturers installation instructions. (Some manufacturers have zero clearance dampers where the damper/ sleeve arrangement has space for expansion). Some installers have sealed the space around the damper with a seal (fire seal) thus impacting the ability of the damper assembly to expand. Some of these seals also expand (such as silicone foam) further adding to the problem. Some may assume they are doing the right thing by installing a seal around a damper assembly but in fact are causing a problem. There are ways to do this if you know what you are doing….