Added Industrial Load on a 4-Inch PE Main
A 4-inch PE 3408 SDR 11 main supplies an industrial park. Inlet pressure at the regulator station is 60 psig; the main is 2.3 miles long and currently delivers 8,500 scfh to 11 customers. The lowest pressure at the last customer is 46 psig at peak, and the tariff minimum is 35 psig. A new customer requests 2,000 scfh at the same peak. Historical pressure-drop data show 14 psi drop at 8,500 scfh. For this Reynolds range, use the approximation that pressure drop varies with flow to the 1.85 power. The utility can either add a 2-inch parallel loop for the last 4,000 ft or raise the regulator setpoint to 65 psig. The regulator station has a 70 psig maximum allowable operating pressure. Evaluate the options and state what additional data you need before committing.
1. Estimate the end pressure at the last customer after the new load is added, assuming the historical drop and exponent are valid.
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New peak flow = 8,500 + 2,000 = 10,500 scfh. Flow ratio = 10,500/8,500 = 1.235. Pressure-drop ratio = 1.235^1.85 = 1.48. New drop = 14 × 1.48 = 20.7 psi. With the station at 60 psig, the estimated end pressure = 60 − 20.7 = 39.3 psig. That is above the 35 psig tariff minimum by 4.3 psi, but the margin is thin and the estimate is only as good as the historical drop and the exponent. The last-customer pressure must be confirmed at the meter inlet, not just on the main.
2. Compare the parallel-loop option with raising the regulator setpoint to 65 psig. Which is technically preferable, and why?
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Raising the setpoint to 65 psig gives an estimated end pressure of about 44.3 psig and solves the immediate pressure issue, provided the station and downstream equipment are rated for it. The station MAOP is 70 psig, so 65 psig is within the station limit, but the utility must still check regulator capacity, relief settings, and customer meter/regulator ratings. A 2-inch parallel loop over the last 4,000 ft increases capacity in that section only; the equivalent capacity gain is roughly 15–20% for that segment, while the new load is 23.5% higher. Because 8,000 ft of the main remains unlooped, the loop alone may not restore the full margin. The lower-risk near-term choice is a setpoint increase with field verification; the loop is better if future load growth is expected.
3. What additional information should be gathered before selecting a final remedy?
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Additional data: actual elevation profile and highest customer, customer meter inlet pressures at peak, current regulator performance curve and station MAOP documentation, pipe internal condition and actual inside diameter, whether 46 psig was measured at the main or after the customer regulator, future load commitments, and the relief-valve settings on customer services. Also verify the 14 psi drop was taken at the same flow and temperature basis. Without these, either option could fail at the coldest peak day.