There has been a lot of confusion about VGB compliant BLOCKABLE & UNBLOCKABLE DRAIN COVERS.
While the following national standards pertain to public/commercial pools, we advocate the same standards for residential vessels. We do not discern a difference between the requirement for a safe hydraulic standards of private VS public/commercial vessels. We feel that private pools should be just as safe as commercial pools when it comes to suction entrapment and safety. After all, there are many more private pools than public pools on this planet!
While the MAXIMUM INDUSTRY STANDARDS allow 6 FPS, we advocate and promote a MAXIMUM SUCTION LINE VELOCITY OF 4 FPS IN THE TRUNK LINE and 2 FPS MAXIMUM THROUGH A BRANCH SUCTION LINE OR A DRAIN INLET (1 FPS when 1.5 FPS is the maximum standard).
First, let's start with an excerpt from a channel drain cover installation manaul:
WATER VELOCITY AND FLOW RATES -
The maximum water velocity through drain covers is limited by some local regulations, for example some state health departments limit the velocity through public pool drain covers to 1½ feet per second. This velocity limit is lower than the flow rating provided by the ANSI/APSP-16 2011 certification, therefore the local limit applies and must be followed. Never exceed the flow rating listed on the cover even if the local code does not provide a velocity limit.
PIPING – GENERAL
For new installations, the piping between drains must be sized to limit the velocity to six feet per second. This limit applies to the
branch piping and all fittings between multiple outlets and the tee leading back to the pump. If code requires a lower water velocity,
comply with the code. See the chart below for information on pipe size flow ratings at six feet per second.
HYDRAULIC SYSTEM SPECIFICATIONS -
ACCEPTABLE PIPE SIZE FOR MAXIMUM RECOMMENDED SYSTEM FLOW RATE PER APSP-7 (6 feet/second in the branch line):
Pipe Size 1½" 2" 2½" 3" 4"
Flow Rate in GPM 45 80 110 160 230 (rounded down)
This is where the confusion lies - blockable drains & un-blockable drains and the relationship of line velocities.
First, Blockable drains:
Though a drain cover may be rated for 10,000 GPM, it is REALLY limited by the MAXIMUM LINE VELOCITY of the attached pipes. On dual drain systems, in the event one drain is blocked, the REMAINING lines must be capable of sustaining 100% of the flow BELOW the 6 FPS maximum. This means, that during normal operation the flow rate through a "blockable drain" must be 3 FPS or less (when a drain becomes blocked the line velocity through the remaining drain will double).
BLOCKABLE DRAIN EXAMPLES:
1) A pump is connected to a 4" trunk line, which is split into (2) 3" branch lines. The 4" line flows 234 GPM @ 6 FPS, while the 3" branch line flows 68 GPM @ 3 FPS. 68 GPM X 2 = 136 GPM. The maximum flow rate of that system is 136 GPM. When one drain is blocked the maximum line velocity of the remaining 3" branch line is limited to 136 GPM - therefore that is the MAXIMUM to be drawn through the 4" trunk line.
2) A pump is connected to a 4" trunk line, which is split into (2) 4" branch lines. The 4" line flows 234 GPM @ 6 FPS, while the 4" branch lines flows 117 GPM @ 3 FPS. The maximum flow rate of that system is 234 GPM. When one drain is blocked the maximum line velocity of the remaining 4" branch line is limited to 234 GPM @ 6 FPS - therefore that is the MAXIMUM that may also be drawn through the 4" trunk line as well.
To select a compliant cover, one would merely select a drain cover that is equal to or above the above GPM flow rate at 3 FPS. If the local standard was 1.5 FPS through the cover, they you'd look for a cover with 2X the GPM flow rating of the above system maximums.
THEREFORE, for blockable drains, the maximum line velocity at the pipes connected directly below the cover are limited to 3 FPS maximum (or lower). In communities with 1.5 FPS standards, the plumbing would have to be split again or upsized to get to 1.5 FPS.
Now, onto UNBLOCKABLE DRAINS:
They are rated using TWO CRITERIA: GPM @ FPS.
Most states limit the FPS flowrate through an UN-BLOCKABLE COVER TO 3 FPS.
Some states (Texas for example) have lowered the FPS through un-blockable covers to 1.5 FPS.
The rating for an un-blockable cover will look like this: 300 GPM at 3 FPS.
So, if you draw 600 GPM through the cover, the line velocity would be 6 FPS - exceeding the rating of the cover. 150 GPM through the cover, would bring the line velocity down to 1.5 FPS (to comply with states that have de-rated such covers).
UN-BLOCKABLE DRAIN EXAMPLES:
1) An un-blockable channel drain is rated at 300 GPM @ 3 FPS. It is connected to a 4" trunk line, that is split into (2) 2.5" branch lines. The 2.5" branch lines are connect to 2 inlets on the bottom of the channel drain.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 300 GPM @ 3 FPS
The line velocity of the attached 2.5" pipes are the limiting factors @ 88 GPM each X 2 = 176 GPM.
The maximum flowrate of this system is 176 GPM.
2) An un-blockable channel drain is rated at 300 GPM @ 3 FPS. It is connected to a 4" trunk line & manifold, that is split into (3) 2.5" branch lines. The 2.5" branch lines connect to 3 inlets on the bottom of the channel drain.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 300 GPM @ 3 FPS
Though the line velocity of the (3) attached 2.5" pipes would allow 264 GPM (e.g. 88 GPM each X 3 = 264 GPM), that would exceed the maximum line velocity of the 4" trunk line at 234 GPM.
The maximum flowrate of this system is 234 GPM @ 6 FPS.
3) An un-blockable channel drain is rated at 300 GPM @ 3 FPS. It is connected to a 4" trunk line & manifold, that is split into (2) 2.5" branch lines. The 2.5" branch lines connect to 2 inlets on the bottom of the channel drain. However, the local authority's standard specifies 1.5 FPS for un-blockable drains.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 150 GPM @ > 1.5 FPS < (a slower standard).
Though the line velocity of the (2) attached 2.5" pipes would allow 176 GPM (e.g. 88 GPM each X 2 = 176 GPM), that would exceed the maximum rating of the un-blockable cover at 1.5 FPS of 150 GPM.
The maximum flowrate of this system is 150 GPM.
Sidenote: the trunk line could not be downsized to 3", because 150 GPM would exceed 6 FPS in a 3" pipe.
4) An un-blockable channel drain is rated at 316 GPM @ 3.9 FPS when floor mounted. It is connected to a 4" trunk line & manifold, that is split into (3) 2.5" branch lines. The 2.5" branch lines connect to 3 inlets on the bottom of the channel drain. However, the local authority's standard specifies 3 FPS for un-blockable drains (therefore, we must solve for the GPM rating at 3 FPS).
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: XXX GPM @ > 3 FPS
316 GPM ÷ 3.9 FPS = 81 GPM/1 FPS
81 GPM X 3 GPM (our standard in this case) = 243 GPM @ 3 FPS
Though the line velocity of the (3) attached 2.5" pipes would allow 264 GPM (e.g. 88 GPM each X 3 = 264 GPM),
and the limit through the drain cover would allow 243 GPM @ 3 FPS,
BOTH OF THOSE FLOWRATES would exceed the maximum line velocity of the 4" trunk line at 234 GPM.
Therefore, the maximum flowrate of this system is 234 GPM @ 6 FPS.
Note the differences in line velocities of the piping attached to bloackable VS un-blockable drains:
Blockable Covers:
The maximum line velocity at the pipe connected below the cover is limited to 3 FPS.
Un-blockable Covers:
The maximum line velocity at the pipe connected below the cover is limited to 6 FPS.
While these are the MAXIMUM INDUSTRY STANDARDS, we advocate and promote a MAXIMUM LINE VELOCITY OF 4 FPS IN THE TRUNK LINE and 2 FPS MAXIMUM THROUGH A BRANCH LINE OR A DRAIN INLET (1 FPS when 1.5 FPS is the maximum).
Hopefully, through these examples, you have come to realize that JUST BECAUSE A COVER HAS A MANUFACTURER'S "FLOW RATING," it is the ATTACHED PIPING SYSTEM THAT DICTATES the MAXIMUM GPM through a plumbing system.
Paolo Benedetti
Aquatic Artist, Watershape Consultant, Expert Witness
"Creating water as art."™
Aquatic Technology Pool and Spa
©www.aquatictechnology.com
While the following national standards pertain to public/commercial pools, we advocate the same standards for residential vessels. We do not discern a difference between the requirement for a safe hydraulic standards of private VS public/commercial vessels. We feel that private pools should be just as safe as commercial pools when it comes to suction entrapment and safety. After all, there are many more private pools than public pools on this planet!
While the MAXIMUM INDUSTRY STANDARDS allow 6 FPS, we advocate and promote a MAXIMUM SUCTION LINE VELOCITY OF 4 FPS IN THE TRUNK LINE and 2 FPS MAXIMUM THROUGH A BRANCH SUCTION LINE OR A DRAIN INLET (1 FPS when 1.5 FPS is the maximum standard).
First, let's start with an excerpt from a channel drain cover installation manaul:
WATER VELOCITY AND FLOW RATES -
The maximum water velocity through drain covers is limited by some local regulations, for example some state health departments limit the velocity through public pool drain covers to 1½ feet per second. This velocity limit is lower than the flow rating provided by the ANSI/APSP-16 2011 certification, therefore the local limit applies and must be followed. Never exceed the flow rating listed on the cover even if the local code does not provide a velocity limit.
PIPING – GENERAL
For new installations, the piping between drains must be sized to limit the velocity to six feet per second. This limit applies to the
branch piping and all fittings between multiple outlets and the tee leading back to the pump. If code requires a lower water velocity,
comply with the code. See the chart below for information on pipe size flow ratings at six feet per second.
HYDRAULIC SYSTEM SPECIFICATIONS -
ACCEPTABLE PIPE SIZE FOR MAXIMUM RECOMMENDED SYSTEM FLOW RATE PER APSP-7 (6 feet/second in the branch line):
Pipe Size 1½" 2" 2½" 3" 4"
Flow Rate in GPM 45 80 110 160 230 (rounded down)
This is where the confusion lies - blockable drains & un-blockable drains and the relationship of line velocities.
First, Blockable drains:
Though a drain cover may be rated for 10,000 GPM, it is REALLY limited by the MAXIMUM LINE VELOCITY of the attached pipes. On dual drain systems, in the event one drain is blocked, the REMAINING lines must be capable of sustaining 100% of the flow BELOW the 6 FPS maximum. This means, that during normal operation the flow rate through a "blockable drain" must be 3 FPS or less (when a drain becomes blocked the line velocity through the remaining drain will double).
BLOCKABLE DRAIN EXAMPLES:
1) A pump is connected to a 4" trunk line, which is split into (2) 3" branch lines. The 4" line flows 234 GPM @ 6 FPS, while the 3" branch line flows 68 GPM @ 3 FPS. 68 GPM X 2 = 136 GPM. The maximum flow rate of that system is 136 GPM. When one drain is blocked the maximum line velocity of the remaining 3" branch line is limited to 136 GPM - therefore that is the MAXIMUM to be drawn through the 4" trunk line.
2) A pump is connected to a 4" trunk line, which is split into (2) 4" branch lines. The 4" line flows 234 GPM @ 6 FPS, while the 4" branch lines flows 117 GPM @ 3 FPS. The maximum flow rate of that system is 234 GPM. When one drain is blocked the maximum line velocity of the remaining 4" branch line is limited to 234 GPM @ 6 FPS - therefore that is the MAXIMUM that may also be drawn through the 4" trunk line as well.
To select a compliant cover, one would merely select a drain cover that is equal to or above the above GPM flow rate at 3 FPS. If the local standard was 1.5 FPS through the cover, they you'd look for a cover with 2X the GPM flow rating of the above system maximums.
THEREFORE, for blockable drains, the maximum line velocity at the pipes connected directly below the cover are limited to 3 FPS maximum (or lower). In communities with 1.5 FPS standards, the plumbing would have to be split again or upsized to get to 1.5 FPS.
Now, onto UNBLOCKABLE DRAINS:
They are rated using TWO CRITERIA: GPM @ FPS.
Most states limit the FPS flowrate through an UN-BLOCKABLE COVER TO 3 FPS.
Some states (Texas for example) have lowered the FPS through un-blockable covers to 1.5 FPS.
The rating for an un-blockable cover will look like this: 300 GPM at 3 FPS.
So, if you draw 600 GPM through the cover, the line velocity would be 6 FPS - exceeding the rating of the cover. 150 GPM through the cover, would bring the line velocity down to 1.5 FPS (to comply with states that have de-rated such covers).
UN-BLOCKABLE DRAIN EXAMPLES:
1) An un-blockable channel drain is rated at 300 GPM @ 3 FPS. It is connected to a 4" trunk line, that is split into (2) 2.5" branch lines. The 2.5" branch lines are connect to 2 inlets on the bottom of the channel drain.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 300 GPM @ 3 FPS
The line velocity of the attached 2.5" pipes are the limiting factors @ 88 GPM each X 2 = 176 GPM.
The maximum flowrate of this system is 176 GPM.
2) An un-blockable channel drain is rated at 300 GPM @ 3 FPS. It is connected to a 4" trunk line & manifold, that is split into (3) 2.5" branch lines. The 2.5" branch lines connect to 3 inlets on the bottom of the channel drain.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 300 GPM @ 3 FPS
Though the line velocity of the (3) attached 2.5" pipes would allow 264 GPM (e.g. 88 GPM each X 3 = 264 GPM), that would exceed the maximum line velocity of the 4" trunk line at 234 GPM.
The maximum flowrate of this system is 234 GPM @ 6 FPS.
3) An un-blockable channel drain is rated at 300 GPM @ 3 FPS. It is connected to a 4" trunk line & manifold, that is split into (2) 2.5" branch lines. The 2.5" branch lines connect to 2 inlets on the bottom of the channel drain. However, the local authority's standard specifies 1.5 FPS for un-blockable drains.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 150 GPM @ > 1.5 FPS < (a slower standard).
Though the line velocity of the (2) attached 2.5" pipes would allow 176 GPM (e.g. 88 GPM each X 2 = 176 GPM), that would exceed the maximum rating of the un-blockable cover at 1.5 FPS of 150 GPM.
The maximum flowrate of this system is 150 GPM.
Sidenote: the trunk line could not be downsized to 3", because 150 GPM would exceed 6 FPS in a 3" pipe.
4) An un-blockable channel drain is rated at 316 GPM @ 3.9 FPS when floor mounted. It is connected to a 4" trunk line & manifold, that is split into (3) 2.5" branch lines. The 2.5" branch lines connect to 3 inlets on the bottom of the channel drain. However, the local authority's standard specifies 3 FPS for un-blockable drains (therefore, we must solve for the GPM rating at 3 FPS).
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: XXX GPM @ > 3 FPS
316 GPM ÷ 3.9 FPS = 81 GPM/1 FPS
81 GPM X 3 GPM (our standard in this case) = 243 GPM @ 3 FPS
Though the line velocity of the (3) attached 2.5" pipes would allow 264 GPM (e.g. 88 GPM each X 3 = 264 GPM),
and the limit through the drain cover would allow 243 GPM @ 3 FPS,
BOTH OF THOSE FLOWRATES would exceed the maximum line velocity of the 4" trunk line at 234 GPM.
Therefore, the maximum flowrate of this system is 234 GPM @ 6 FPS.
Note the differences in line velocities of the piping attached to bloackable VS un-blockable drains:
Blockable Covers:
The maximum line velocity at the pipe connected below the cover is limited to 3 FPS.
Un-blockable Covers:
The maximum line velocity at the pipe connected below the cover is limited to 6 FPS.
While these are the MAXIMUM INDUSTRY STANDARDS, we advocate and promote a MAXIMUM LINE VELOCITY OF 4 FPS IN THE TRUNK LINE and 2 FPS MAXIMUM THROUGH A BRANCH LINE OR A DRAIN INLET (1 FPS when 1.5 FPS is the maximum).
Hopefully, through these examples, you have come to realize that JUST BECAUSE A COVER HAS A MANUFACTURER'S "FLOW RATING," it is the ATTACHED PIPING SYSTEM THAT DICTATES the MAXIMUM GPM through a plumbing system.
Paolo Benedetti
Aquatic Artist, Watershape Consultant, Expert Witness
"Creating water as art."™
Aquatic Technology Pool and Spa
©www.aquatictechnology.com
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If you want to ask a question of Paolo Benedetti, you may email him at: info@aquatictechnology.com