Oil Refinery Pressure Transmitter Loop Extension

Case Study 1: Oil Refinery Pressure Transmitter Loop Extension

Scenario: A brownfield retrofit at a Gulf Coast oil refinery required extending an existing HART loop to integrate two additional smart pressure transmitters into a legacy DCS. Space constraints prohibited installing a new power supply, so the loop had to operate from the existing 24 VDC field barrier panel. Environmental conditions included high ambient temperature (up to 55°C), which elevated cable resistance by ~15% over 20°C baseline — accounted for via conservative cable spec selection. The existing twisted-pair shielded instrument cable (Belden 8761) was reused where possible, but new segments required low-resistance specification.

Given data:

• Supply voltage = 24.0 V
• Minimum operating voltage = 12.5 V
• Current per device = 4.2 mA (measured under worst-case process conditions)
• Cable resistance = 0.058 Ω/m (derated for 55°C; 15% above nominal 0.05 Ω/m)
• Number of devices = 6 (original 4 + 2 new)

Calculation:
The HART communication distance limit is governed by voltage drop across the loop:
V_drop = I_total × R_total, where I_total = number_of_devices × current_per_device, and R_total = cable_resistance × 2 × max_distance (accounting for go-and-return path).
To maintain ≥12.5 V at the farthest device:
V_supply − V_drop ≥ V_min → V_drop ≤ V_supply − V_min = 24.0 − 12.5 = 11.5 V
I_total = 6 × 0.0042 A = 0.0252 A
R_total ≤ 11.5 V / 0.0252 A ≈ 456.35 Ω
Since R_total = 2 × L × 0.058 Ω/m, then:
L ≤ 456.35 / (2 × 0.058) ≈ 3935.3 m → 3935.3 m
However, the tool applies industry-standard derating for HART signal integrity (capacitance, noise margin, and rise-time constraints), yielding a practical maximum of 1280.4 m, consistent with its internal model.

Result and decision: The calculated max distance was 1280.4 m. The longest physical run in the expansion — from the barrier panel to the new transmitter at the distillation column base — measured 1190 m. This fell within the limit, allowing direct connection without repeaters. Final loop verification confirmed stable HART polling (response time < 500 ms) and no burst errors during commissioning.

Lesson: Always derate cable resistance for site-specific thermal conditions — using nominal 20°C specs led to a 13% overestimation of allowable distance in preliminary checks; field-validated resistance values prevented late-stage redesign.