Authored by: Edited by Key points: • Tubing that’s too big increases cost, space requirements, and weight. • Size tubing based on material, diameter, and wall thickness to meet pressure and flow requirements. Resources: |
A tube that is too small raises fluid velocity, which has many detrimental effects. In suction lines, it leads to cavitation that starves and damages pumps. In pressure lines, it causes excessive friction losses and turbulence, both resulting in high pressure drops and heat generation. High heat, in turn, accelerates wear in moving parts and ages seals and hoses. The end results are shorter component life and wasted energy and, therefore, low levels of efficiency.
On the other hand, tubing that’s too large unnecessarily increases costs. It can also consume valuable real estate that makes it more difficult to fit tubing into confined spaces and restricts engineers in configuring adjacent equipment and components. And oversized tubing can simply be harder and more time consuming to install. It also weighs more than necessary, and that hurts fuel consumption on mobile equipment.Matching tubing to an application involves selecting the right material and determining optimum OD and wall thickness. Here’s a simple procedure for sizing tubes.
Flow diameter
The first step is to determine required flow diameter. The accompanying “Recommended flow diameter” table gives guidelines for specific flow rates and types of line.
The table is based on the following recommended flow velocities:
• Pressure lines — 25 fps or 7.62 meters/sec.
• Return lines — 10 fps or 3.05 meters/sec.
• Suction lines — 4 fps or 1.22 meters/sec.
If flow velocities differ from these, calculate the required flow diameter based on:
d = 0.64(Q/V)^{0.5}
where d = tube ID, in.; Q = flow, gpm; and V = velocity, fps.
For metric units:
d_{m} = 4.61(Q_{m}/V_{m})^{0.5}
where d_{m} = tube ID, mm; Q_{m} = flow, lpm; and V_{m} = velocity, meters/sec.
Diameter and thickness
Next determine tube OD and wall thickness. Using the “Pressure ratings” table, find the diameter and thickness combination that satisfies the following two conditions:
1. Has recommended design pressure that equals or exceeds maximum operating pressure.
2. Provides tube ID that equals or exceeds the required flow diameter determined earlier.
Design pressures in the table are based on a severity of service rating “A” (design or safety factor of 4) as listed in the “Design and derating factors” table.
In more-severe operating conditions, multiply values in the pressure-ratings table by the appropriate derating factors before determining the tube OD and wall thickness combination. Contact a reputable tubing supplier when in doubt.
Allowable stress levels and the underlying specifications used to arrive at the pressure ratings are given in the “Design stress ratings” chart. Values are for fully annealed tubing.
Or calculate design pressure based on Lame’s equation,
P = S((D^{2} – d^{2})/(D^{2} + d^{2}))
where D = tube OD, in.; d = tube ID, in. (D – 2T); P = recommended design pressure, psi; S = allowable stress for a design factor of 4, psi; and T = tube wall thickness, in.
For thin wall tubes (D/T ≥ 10) the following equation may be used:
P = 2ST/D.
The design factor is generally applied to the material’s ultimate strength (or tubing burst pressure) to provide a margin of safety against unknowns in material and operating conditions. Apply the derating factors listed here directly to values in the pressure ratings table to determine maximum recommended working pressures. That is, multiply values in the table by the derating factors.
Besides severity of service, high operating temperatures also reduce allowable working pressure in tubing. Temperature-derating factors for various tube materials are given in the accompanying table. Where applicable, apply derating factors for severity of service and temperature to the design pressure values (from the table) to calculate the maximum recommended working pressure. For example, the combined derating factor for 316SS tubing for B (severe) service and 500°F operation is 0.67 × 0.9 = 0.60.
Selection example
Here’s an example of the tube-sizing process for pressure, return, and suction lines for a hydraulic power unit using petroleum-based hydraulic fluid. Operating temperature range is –20 to 140°F, maximum operating pressure is 3,500 psi, maximum flow rate through each line is 10 gpm, and severity of service is A (normal).
1. Select tube material. Carbon steel, C-1010, is an economical choice. It is suitable for petroleum fluids, has an operating temperature range of –65 to 500°F, and meets maximum operating pressure requirements. Tubing suppliers provide data on many other materials, such as alloy and stainless steels, copper, aluminum and Monel, as well as plastics such as nylon and PVC.
2. Size the tube. According to the Flow diameter table, recommended IDs for lines at 10 gpm flow rate are: 0.405 in. for the pressure line, 0.639 in. for the return line, and 1.012 in. for the suction line.
3. Pressure rating. Now, using the pressure-rating table, find tubes with IDs equal to or greater than the above flow diameters, and wall thicknesses appropriate for design pressures of 3,500-psi minimum for the pressure line and about 500 psi for return and suction lines. Because derating factors for severity of service and maximum operating temperature are both 1.0, design pressure values in the tables do not need to be reduced.
Next, match tube IDs and pressures in the tables for these conditions. For the pressure line, select 0.62-in. OD × 0.083-in. wall tubing. The 0.095 and 0.109-in.-thick wall would also be satisfactory if 0.083-in. wall is not readily available.
For the return line, either 0.75 × 0.035 in. or 0.75 × 0.049 in. would meet requirements. Also note that the type of fitting can affect tubing selection. Here, for instance, SAE J514 flareless fittings (Parker Ferulok) would require 0.75 × 0.065 in. because 0.065 in. is the thinnest wall recommended for this fitting with 0.75-in. tubing. This would reduce flow diameter about 3% below recommended, but is still in the acceptable range. The alternative would be 0.87 OD × 0.072-in. wall tubing, which is excessively large. Fitting manufacturers provide tube thickness recommendations for various fittings and sizes.
For the suction line, use 1.25 OD × 0.049 to 0.083-in. wall tube for SAE J514 37° flare (Parker Triple-Lok) or SAE J1453 O-ring face seal (Parker Seal-Lok) fittings and 1.25 OD × 0.095-in. wall tube for flareless fittings.
One final consideration is choosing the right wall thickness for bent tubing. If bending without a mandrel, then wall thickness of less than 7% of tube OD is not recommended.