Loss of steam through steam traps accounts for significant energy waste at many plants. Quantifying the extent of the loss often poses difficulties. Indeed, to date, no simple-to-use predictive tool can accurately estimate the actual rates of lost steam as a function of steam line pressure and saturation temperature of sub-cooled condensate. Here, however, we present an accurate and reliable method that requires fewer computations than conventional approaches.
The tool predicts the condensate flow, Q, in kg/h, via a simple equation:
Ln(Q) = a + b/P + c/P2 + d/P3 (1)
where P is steam line pressure in kPa (abs) and the four coefficients relate to the sub-cooled condensate's saturation temperature, T, in K, via:
a = A1 + B1T + C1T2 + D1T3(2)
b = A2 + B2T + C2T2 + D2T3 (3)
c = A3 + B3T + C3T2 + D3T3 (4)
d = A4 + B4T + C4T2 + D4T3 (5)
The optimum tuned coefficients (A, B, C and D) appear in Table 1. They cover condensate flow rates for steam traps with a flow capability, CV, of 1 in data reported in the 4th edition of C. R. Branan's "Rules of Thumb for Chemical Engineers," Gulf Publishing (2005).
In the next step, the result from Eq. 1 is used to estimate actual loss of steam. Equation 6 gives the corrected steam-trap flow factor:
FC = QC/Q (6)
Equation 7 then predicts actual steam loss, SA, as a function of trap inlet pressure, in kPa (abs), and the corrected flow factor:
SA = (0.093P – 9.4589) FC (7)
Figure 1 compares the tool's predictions of condensate flow rates in steam traps with CV = 1 as a function of steam line pressure and saturation temperature of sub-cooled condensate with data reported by Branan. The results agree well with these data. Figure 2 depicts the tool's performance for estimating condensate flow rates as a function of steam line pressure and saturation temperature of sub-cooled condensate. Figure 3 compares predicted steam loss as a function of inlet pressure with Branan's data. Table 2 highlights the very good agreement with the reported data — the average absolute deviation is 2.87%.