Aquatic Technology Pool & Spa

The Whole Equation

The Whole Equation:Installing a VGB-approved cover isn’t all it takes to safeguard a drain.

By Rebecca Robledo

Glass Tile Mosaics - Tessellated StressSince its enactment in 2008, a lot has been written about the Virginia Graeme Baker Pool and Spa Safety Act. Yet, in my travels as an expert witness, I continually see indications that the law and how it should be implemented are not understood.

The confusion seems to lie in the complexities of components selection and hydraulic design.

Though drain sumps, channels and covers that comply with VGB list a maximum flow rating in gallons per minute, it does not mean that the pipe connections below that cover are permitted to flow at that rating.

There are two criteria that apply to drains to make them safe. The first is the rating on the cover, which states the maximum flow rate allowed. If the drain is mounted on the side wall, the allowable rate is generally a little lower than if it’s on the floor. Manufacturers base this data on the perforations or total cumulative square inches of open space in the grate. Those ratings generally allow a couple hundred gpm of flow.

But that number is determined without consideration of the second criterion — line velocity of the pipes underneath the sump. VGB says plumbing that connects to the drain sump should have a line velocity of 3 feet per second or less in an unblocked condition. Most channel drains come with 2-inch connection ports. At 3 fps, a 2-inch pipe is limited to 32 gpm. (Some jurisdictions and health departments, such as the Texas State Department of Health, limit the line velocity through the branch suction lines to 1.5 fps.)

That’s where the disconnect comes in: The high flow rates stamped on drain covers don’t match the gpm that accompanies compliant velocities.

Over the decades, I’ve looked at pools by award-winning builders who’ve gotten in a dispute with the owners. One of the things we do when we come out is drain the pool, get in there, take the drain covers off and look underneath. We’re trying to figure out what’s happening with the hydraulics, calculate the total dynamic head of the system and generally determine what’s going on. Line velocities are usually one of the first things we check because we know pipes are so frequently undersized.

When I point this out, builders often defend themselves by saying, “Well, the drain manufacturer says that the [allowable] drain flow is 800 gpm.” That may be true, but if it only has a 2-inch pipe connected to it, the speed limit for that pipe, at 3 fps, is still only 32 gpm. So I explain to the builder that he or she will need to install multiple inlets.

Based on what I’ve seen, too many builders are picking the drain covers and letting it dictate how they should build, when it should be the other way around.

It is a mistake to focus solely on the flow rating of the cover, and not the line velocity through the pipe. Some think that because the manufacturer tells them they can flow, say, 400 gallons per minute through the drain cover, they can put that volume through the 2-inch pipe underneath it. That isn’t the case. The pipe has to flow at 3 feet per second maximum. In fact, I think that’s what it should say on the lids: “Shall not exceed x gallons per minute at 3 feet per second.”

It’s similar to the process of choosing a pump — you design the hydraulic system, then select the pump last. It should be the same thing with drain grates — it should be the last thing that you pick based on the speed limit that you establish at the pipe.

Designers should remember something else: The maximum flow rating on the drain cover should be cut in half to start, in case one of the drains becomes blocked. If that happens, flow through the others should not exceed 6 fps. Then pick a drain cover that will allow that flow rate through the remaining covers.

I’ve seen builders connect a 2-inch suction line to a 3-horsepower pump, and then split it into two 2-inch suctions and think, “Okay, that pump is flowing at 200 gpm.” Since the two grates are rated at, say, 150 gpm, the builder says, “Okay, 150 plus 150 is 300, but my pump only generates 200 gpm. I’m more than safe.” And away they go. But the line velocity is too high. If one of the suction inlets is blocked, the line velocity through the remaining inlets now exceeds the 6-fps goal for a blocked drain. In that case, it also would exceed the flow rate of that drain cover.

So some contractors aren’t doing the math. They’re not saying, “If one drain gets blocked, what happens to the hydraulics in the remaining drain? Is it now exceeding the speed limit? Is it now exceeding the flow rate for the drain cover?”

But the drain cover rating does allow the grouping of different suction systems. Multiple branch lines from different pumping systems may be grouped together under a cover, until their cumulative gpm flow rate meets the cover’s rating. This allows multiple system inlets to share a drain cover.

The simple solution to reduce line velocities is to use larger pipe. When doing that, always make sure the clearance between the pipe and the backside of the cover meets VGB specifications. For a 4-inch inlet, there generally needs to be a 6-inch clearance between the pipe and the backside of the cover. For 2-inch pipe, the clearance usually must be 3 inches.

Pre-manufactured sumps can help eliminate the margin of error for that spacing, because it’s already been determined at the factory. The producer has already made their molds so you have that minimum clearance built in. Many contractors prefer to carve the sumps in the gunite because it’s more economical and less of a hassle than the pre-manufactured versions, which must be pressurized. But it may be worth the effort.