ARMOR-CLAD©  FLEXIBLE PIPE SIZING

Tank Size & Rainfall Amounts

To determine size and number of flexible roof drains for a floating roof tank, the manufacturer and the tank owner (or contractor) analyze the dimensions of the tank, including the lowest roof operating height. The slowest flow rates for any type of drain system can be expected when the roof is at its lowest level. They will also allow for the maximum hourly rainfall that can be expected in that geographical area. Based on these considerations, the manufacturer will recommend a drain system that will meet or exceed the amount of water that could accumulate on the floating roof. Drain sizes typically range from 3 inches on the small end of the spectrum to 8 inch diameters in the largest tanks. API standard 650, the source for aboveground storage tank fabrication in the US and increasingly a popular reference tool used in other parts of the world, specifies roof drain sizing for storage tanks, stating: 'primary roof drains shall not be smaller than NPS 3 for roofs with a diameter less than or equal 36 m (120 ft) or smaller than NPS 4 for roofs with a diameter greater than 36 m (120 ft).' Primary roof drains are differentiated from emergency roof drains, which are designed to operate only if water accumulated on the roof reaches an excessive level. While the guidelines above provide a useful reference point, the number and size of flexible roof drains cannot be determined on tank size alone. The drain system must also be capable of meeting the rainfall amounts in the area. API-650 states that: "primary roof drains shall be sized and positioned to accommodate the rainfall rates specified on the data sheet...while preventing the roof from accumulating a water level greater than design, without allowing the roof to tilt excessively or interfere with its operation.' Tank owners cannot simply install 4 inch drains in tanks over 35 m diameter without considering expected rainfall amounts. For example, a tank with diameter of 79 m (259 ft) may include two 6 inch drains to handle the amount of water that can accumulate on that size roof in that part of the world. Special care must be taken if the tank is located in a geographical area that can experience brief heavy rains, monsoon rain events or tropical storms.

ARMOR-CLAD size specifications
I.D.	WEIGHT PER FOOT	MINIMUM WORKING BEND RADIUS	MAXIMUM WORKING PRESSURE	VACUUM RATING INCHES HG.	STANDARD TEMP. RANGE
3 in	5.5 lb	48 in	150 psi	29	- 40 F to 180 F
4 in	8.4 lb	54 in	150 psi	29	- 40 F to 180 F
6 in	20 lb	54 in	150 psi	29	- 40 F to 180 F
8 in	32 lb	60 in	100 psi	29	- 40 F to 180 F

PIPE LENGTH CALCULATION

Let R = horizontal distance (free space) between rigid pipe flanges, and H = tank height. If R < H, then L = H - R + 1.57 R If R > H, then L = R - H + 1.57 H

armor-CLAD flow rate (examples)
Tank specifications – 100’ diameter x 48’ high
Armor-Clad I.D x Length	Maximum flow rate at low leg position - GPM	Maximum flow rate with roof at highest position - GPM
3” x 56’	119	336
4” x 56’	241	682
6” x 56’	649	1,835
8” x 56’	1302	3,683
The above flow rates were developed using: HO at ambient temps, D ‘Arcy equation for pressure drop