Plans Examiner - understanding TDH , variable pump speed vs drain gpm

[Kelley Drawdy]

I am a plans examiner with our local building department.

My knowledge about residential pools/equipment is VERY limited and I am trying to learn.

The key item have been asked to look at is the main drain - and how it is sized to "keep up" with the maximum speed of the pump given the calculated TDH. (most contractors use the simplified TDH)

I've been instructed to take the calculated simplified TDH, use the performance curve for the pump and the max speed of the pump to check the size of the main drain that has been specified. The curve below shows data from an application with a TDH of 10.6 and a drain with gpm of 150. The way it has been explained to me - based on the calculation - the main drain is not sufficient.

Does this sound correct?
THANKS in advance for anyone willing to help.

 

[Jimrahbe]

Kelly,

Let's see if these guys can chime in...

Calling @mas985

Calling @JamesW

Thanks,

Jim R.

 

[JamesW]

Is this a new or existing pool?

There should be dual main drains, so you should have 2 at 150 gpm = 300 gpm, correct?

What size is the pipe going to the main drains?

I would need to see your local code to know how to properly design a system.

Does your code require the main drains to be able to handle the full maximum possible flow from the pump?

 

[JamesW]

The curve below shows data from an application with a TDH of 10.6 and a drain with gpm of 150.

10.6 feet of head at 150 gpm is not a realistic head loss for a residential pool system.

I would need to see the system and the calculations to double check.

 

[Kelley Drawdy]

Is this a new or existing pool?

There should be dual main drains, so you should have 2 at 150 gpm = 300 gpm, correct?

What size is the pipe going to the main drains?

I would need to see your local code to know how to properly design a system.

Does your code require the main drains to be able to handle the full maximum possible flow from the pump?

• Application to building department for a new pool.
• 2 drains - and ok - so you DO multiply number of drains x drain gpm for total gpm......
• 2" pipe
• not really looking the design of the system - as a code official we are looking mainly at "safety" as it relates to suction
• yes - main drains need to handle the pump at max speed

 

[Kelley Drawdy]

10.6 feet of head at 150 gpm is not a realistic head loss for a residential pool system.

I would need to see the system and the calculations to double check.

The TDH calcs OFTEN seem off - typically the pool contractor is not filling out the paperwork - it's people that are "trained" in the office - I had someone admit to me that she had no idea what she was doing.

If you have time and would like to take a look at this - you'll see some issues with the calculations.

 

[JamesW]

The code might be that a single drain can handle the total flow in case one gets blocked.

This is going to be too much for me to get into at this point.

Maybe someone else can jump in if they have time.

 

[mas985]

I've been instructed to take the calculated simplified TDH, use the performance curve for the pump and the max speed of the pump to check the size of the main drain that has been specified. The curve below shows data from an application with a TDH of 10.6 and a drain with gpm of 150. The way it has been explained to me - based on the calculation - the main drain is not sufficient.

Does this sound correct?
THANKS in advance for anyone willing to help.

No that is not correct. TDH is dependent on flow rate and flow rate is dependent on TDH. It is a system of two equations and two unknowns. Simplified TDH nearly always gives the wrong answer and is a horrible way to go about the calculation. A more rigorous method is required to come up with a better estimate. What is really needed is a list of ALL the equipment. Everything adds to head loss, filters, heaters, valves, fittings, eyeballs, etc. Nothing can be ignored like what simplified TDH does.

Another option that is easier but less accurate is to use an estimate of the plumbing's head curve. For example if most of the equipment is 2" and there is a filter and heater in line with the plumbing, the plumbing curve can be estimated from the following equation:

TDH = 0.0082 * GPM^2

If you plot this over the pump's head curve, you can get an operation point where the two cross (~100 GPM). However, this is a very rough estimate. To get a better estimate, the exact length of every pipe and a count of every fitting and piece of equipment is required. Simplified TDH will likely give you the wrong answer.

But one thing that is often ignored is that with a VS pump, the top speed can be limited to meet any specification.

 

[Kelley Drawdy]

No that is not correct. TDH is dependent on flow rate and flow rate is dependent on TDH. It is a system of two equations and two unknowns. Simplified TDH nearly always gives the wrong answer and is a horrible way to go about the calculation. A more rigorous method is required to come up with a better estimate. What is really needed is a list of ALL the equipment. Everything adds to head loss, filters, heaters, valves, fittings, eyeballs, etc. Nothing can be ignored like what simplified TDH does.

Another option that is easier but less accurate is to use an estimate of the plumbing's head curve. For example if most of the equipment is 2" and there is a filter and heater in line with the plumbing, the plumbing curve can be estimated from the following equation:

TDH = 0.0082 * GPM^2

If you plot this over the pump's head curve, you can get an operation point where the two cross (~100 GPM). However, this is a very rough estimate. To get a better estimate, the exact length of every pipe and a count of every fitting and piece of equipment is required. Simplified TDH will likely give you the wrong answer.

But one thing that is often ignored is that with a VS pump, the top speed can be limited to meet any specification.

Thank you for your reply!

The thought is that a homeowner can "remove" limits of the VS pump.

Code compliance has its root in "safety" - and the potential for danger.

I believe the building department accepts minimums such as simplified TDH to meet "minimum code compliancy".

Thank you for the lesson.

 

[JamesW]

This seems to imply that the pump should be sized such that it cannot provide more than 36 gpm.

Honestly, building departments are horrible at providing clear instructions.

They seem to go out of their way to create a confusing mess of incomprehensible and contradictory instructions that no one can understand or follow.

Most pool builders just figure out what a local inspector wants and do what they are told to.

Most instructions have nothing to do with common sense or logic.