Inquiring PM Engineer readers want to know…many things.



Q. Our local fire official is requiring that we supply some type of hanger support for 1" drops to a suspended ceiling in our building because those drops exceed 12 feet in length. To substantiate his ruling, he has cited NFPA 13, Section 9.2.2.1, which references the requirement that the maximum distance between hangers on one-inch pipe shall not exceed 12 feet. Are the hangers required on these long drops?

A. No, no, no. It has never been the intent of the NFPA standards to require hangers on drops. The requirements you see in Sections 9.2.2 and 9.2.3 in NFPA 13 (2002) apply strictly to horizontal fire sprinkler piping. Only in Section 9.2.5 is there any mention of support for vertical runs. 9.2.5.1 states that “risers shall be supported by pipe clamps or by hangers located on the horizontal connections within 24 inches of the centerline of the riser.” Instances in multi-story buildings are addressed in Section 9.2.5.4, requiring that the “distance between supports for risers shall not exceed 25 feet.”



Section 9.2.3.5.1 of the 2007 edition of NFPA 13 states that the horizontal length of an unsupported armover to a pendent sprinkler drop shall not exceed 24 inches when using steel pipe-unless maximum static pressure exceeds 100 psi, in which case that horizontal length shall not exceed 12 inches.

You may wish to bring up the fact that Section 9.2.3.5, which has been in the standard for years, stipulates that a steel pipe armover to a sprinkler drop can be unsupported provided that it does not exceed 24 inches-regardless of the length of the attached (vertical) drop. If you are sprigging up to a fire sprinkler, that is a different matter altogether. Section 9.3.6.5 mandates that “sprig-ups 4 feet or longer shall be restrained against lateral movement.” This requirement first appeared in the 1994 edition of NFPA 13. At that time, the restriction applied only to sprigs exceeding 8 feet in length. The standard’s appendix details usage of restraining wire to acceptably support long sprigs.



Q. We are specifying materials for a fire sprinkler system installation and were wondering how much more a sprinkler system will cost if we go with galvanized piping. We know the system will last longer being galvanized, but what about the up-front cost?

A. Let me start by quoting from the “Cost Predictions” chapter of a law book titled, Legal Aspects of Architecture, Engineering, and the Construction Process, which asserts that “the design professional contends that cost predictions are educated guesses, the accuracy of which depend to a large part on events beyond the control of the design professional. Clients frequently request cost predictions before many details of the project have been worked out…design professionals point to labor and material costs which can fluctuate wildly.”

Now that the disclaimer is out of the way, I can tell you that currently black steel thinwall piping runs about 52% of the cost of galvanized. Black Schedule 40 piping goes for about 42% the cost of galvanized Schedule 40. Black cast iron threaded fittings are two-thirds the cost of their galvanized counterparts. Normal grooved fittings sell for 60% the cost of galvanized grooved fittings. Twenty years ago, your price increases for galvanizing were only about 45% for pipe and 15% for fittings. Those percentages are much higher today.

Let’s look at a typically installed fire sprinkler system installation (assuming one really exists). Main and branch-line fittings, along with piping, will comprise roughly two-thirds of the total material cost, and slightly less than that if the system is dry. If union labor is involved, labor makes up a large bulk of the selling price. Total material cost (including hangers, valves, sprinklers, flanged fittings, dry valve, compressor, other components) will comprise somewhere in the vicinity of 35% of the total contract. Also note that if galvanized pipe is used on a dry system, the hydraulic advantage realized will usually mean that some piping can be downsized, a reduction in material outlay.

Combining and consuming this cacophony of data, for a $75,000 fire sprinkler installation contract done in black steel and cast iron, my rough estimate would be for a revised total contract of $92,000 if done with all galvanized pipe and fittings. But don’t quote me.

Q. We’ve had a complete occupancy change in our building and need to upgrade design densities for several of our sprinkler systems. Our insurance guy said that this can be accomplished by changing the rating of our 1,500 gpm fire pump from 80 to 150 psi. How much work and expenditure will that entail?

A. A lot depends on the age and type of the fire pump. Availability of parts may be an issue. In some cases, the impeller of the original pump may have been trimmed for customization, so there may now be room to put in a bigger impeller. You will have to consult with the pump manufacturer’s local rep. He will tell you that you cannot really change a condition of service in the field-the pump will have to be modified and tested back at the factory or else you will void its labeling. “Field” changes made to fire pumps break the listing of the unit.

In your case, it appears as though a new motor is in order since the horsepower will have to be increased from 100 hp to 200 hp. This means that you also will have to change the controller and the power supply from the pole.If it’s old, the current controller probably wouldn’t meet current codes anyway. You are going to be dollars ahead if you contract someone to research your existing systems. This will involve an engineering expenditure for his trial-and-error hydraulic calculation exercises. By increasing main or line pipe sizing, adding branch lines, converting tree systems to grids or by changing sprinklers, the odds are good that your existing systems can be modified so that they fall in line with the new design demand. And your existing pump will not go the way of the slide rule.

Q. The Automatic Sprinkler Systems Handbook has an exhibit on page 190 showing three wet system risers, each with an alarm check and control valve. If a “shotgun” riser manifold was used in place of the alarm check, would a check valve be required between each control valve and riser manifold? I read the definition of the system riser in Chapter 3 and it does not include a check valve. The arrangement would have the single check (double-check assembly in Montana) valve for the water supply to the manifold. I have been including the check valves, but please let me know your thoughts or guide me to the applicable code section.

A. The check valve, double-check assembly or any backflow prevention unit is required to: 1) lock pressure into the fire sprinkler system, and 2) prohibit any tainted or contaminated sprinkler water from flowing back into the potable municipal water supply, which feeds the sprinkler system. Each wet-pipe system within the building requires its own indicating valve, main drain assembly, flow switch and pressure switch. Probably a tamper switch will also be required for the gate (control) valve, depending on local enacted legislation. You do not need a check valve on each riser.