Steam

Condensate line sizing with flash steam and steam trap verification

Calculates the flash steam formed after the steam trap by enthalpy balance (IAPWS-IF97), sizes the discharge line by the flash steam velocity (FCI ST-108) and checks whether the trap has differential and capacity in each operating condition: start-up, minimum pressure, minimum load and nominal operation.

When to use

Use it when designing or reviewing the condensate drainage of steam equipment (heat exchanger, jacket, coil, line drip point) that discharges to a pressurized return, gravity return, flash tank, deaerator, pressurized header or pumped-return receiver. The calculator answers two separate questions. The first is whether the discharge line can carry the flash steam, which takes almost the whole pipe section. The second is whether the trap can discharge: the pressure upstream of it must overcome the back pressure (destination pressure, lift head and line losses) in every condition, not only in nominal operation. Start-up, with the equipment near atmospheric pressure and the largest condensate load, and the minimum operating pressure of equipment under modulating control are where the differential is usually missing. The liquid drain line before the trap and the NPSH of a pumped-return pump are out of scope.

What this calculator solves

At the outlet of a steam trap the condensate leaves saturated at the equipment pressure and enters a line at lower pressure. Part of it flashes at once — the flash steam — and that steam, with a specific volume hundreds of times larger than the liquid, takes almost the whole pipe section. A line sized “for water” ends up too small, velocity rises, water hammer, noise and erosion appear, and the back pressure on the trap goes up.

The calculator makes two separate checks: the line (by flash velocity, FCI ST-108) and the trap (differential and capacity in each operating condition).

How the method works

Flow classification. The condensate enthalpy (saturated at the trap pressure, or subcooled if you enter the temperature) is compared with saturated liquid at the destination pressure, by IAPWS-IF97. Together with the declared destination — pressurized return, gravity, flash tank, deaerator, header or pumped return — this sets the nature of the discharge.
Line by flash. With flash, the minimum area is the steam volumetric flow divided by the velocity limit (15.24 m/s by default), plus the liquid area fraction. If the liquid dominates the volume, the liquid velocity (2 m/s, adjustable LOGOS criterion) and its pressure drop are also checked.
Back pressure. Destination pressure + lift head + declared single-phase losses + two-phase ΔP when there is a valid correlation, solved together with the flash. Without a valid correlation, the liquid-phase friction enters as a floor.
Trap by condition. Start-up, minimum pressure, minimum load and nominal operation: differential, load × factor and capacity from the manufacturer curve. The worst state becomes the trap result.

Honest results

The calculator would rather say “not evaluated” than approve with missing data. Capacity is not evaluated without the manufacturer curve. “Meets” is not issued while the back pressure lacks the line two-phase ΔP — because the real differential would be smaller than calculated. Stall and lack of capacity (with a monotonic curve) are conclusive anyway, since the missing loss could only make the result worse. In a partially filled gravity return, the back pressure is the destination pressure, the column and the single-phase losses, and the verdict is issued normally.

Out of scope

The liquid drain line before the trap, the condensate pump NPSH and the two-phase ΔP by correlations without a documented range are not calculated. Pressures are limited to 100 bar(a), the LOGOS Steam product limit (the calculation also requires liquid–vapour saturation).

Formulas and fundamentals

Flash fraction by enthalpy balance x_f = (h_in − h'(P_dest)) / h_fg(P_dest)

Fraction of the condensate flow that becomes steam when it drops from the trap pressure to the destination pressure. h_in = condensate enthalpy at the trap inlet [kJ/kg] (saturated liquid at the trap pressure, or at the entered temperature if the condensate arrives subcooled); h' = saturated liquid enthalpy at the destination [kJ/kg]; h_fg = latent heat at the destination [kJ/kg]. Properties by IAPWS-IF97. If x_f ≤ 0 there is no flash and the line is treated as liquid.

Line area by flash steam velocity (FCI ST-108) A = ṁ_flash·v''(P_dest) / V_lim · [1 + ṁ_liq·v'(P_dest) / (ṁ_flash·v''(P_dest))]

Minimum internal area of the discharge line. ṁ_flash and ṁ_liq = flash steam and liquid flows [kg/s]; v'' and v' = specific volumes of saturated vapour and liquid [m³/kg]; V_lim = flash velocity limit, default 3000 ft/min (15.24 m/s), adjustable. The bracket adds the area taken by the liquid. Properties are taken at the destination pressure, where the volume is largest (conservative side). D_min = √(4A/π).

Steam trap back pressure P_back = P_dest + ρ_l·g·Δz + ΔP_single-phase + ΔP_two-phase(P_back)

Pressure at the trap outlet. P_dest = destination pressure [bar(g)]; ρ_l·g·Δz = condensate column of the lift up to the return high point; ΔP_single-phase = declared losses of fittings and equipment [bar]; ΔP_two-phase = friction of the mixture in the line, which only enters with a correlation of documented application range. Flash and back pressure are solved together. A downward run only lowers P_back with a continuous liquid column (flooded line or no flash), and never below the vapour pressure of the liquid; in a non-flooded two-phase discharge Δz = 0 is adopted.

Liquid-phase friction as a floor of the two-phase loss ΔP_L,ref = f·(L/D)·ρ_l·v_SL²/2 ≤ ΔP_two-phase (φ_L² ≥ 1)

With flash and no valid two-phase correlation, the liquid-phase loss at the liquid superficial velocity v_SL (Darcy–Weisbach, saturated liquid at the line pressure) enters the back pressure as a lower bound, because the Lockhart–Martinelli multiplier φ_L² is never below 1. Without flash, the same expression is the real friction of the liquid line. Fittings are not included.

Trap differential and required load ΔP_trap = P_trap − P_back ; required load = load × F

ΔP_trap [bar] is the differential available at the trap in each condition. F = load factor for trap selection, 2 by default (market practice, adjustable, ≥ 1). The required load is compared with the capacity read from the manufacturer curve (ΔP × kg/h) at that condition's ΔP.

Liquid velocity when the liquid dominates the volume v_L = ṁ_liq·v' / A ≤ 2 m/s (if Q_liq > Q_flash)

When the liquid volume exceeds the flash volume (little or no flash), sizing by FCI ST-108 is not enough: the liquid velocity is checked against 2 m/s (LOGOS design criterion, adjustable) and the liquid pressure drop by Darcy–Weisbach.

Standards & methods

IAPWS-IF97 (IAPWS R7-97(2012)) — enthalpies, specific volumes and saturation
IAPWS R12-08 / R15-11 — viscosity of water and steam (industrial form), used in friction
FCI ST-108 (2016) — Condensate Line Sizing, Fluid Controls Institute (flash velocity)
IAPWS R1-76(2014) — surface tension (used only in the informative two-phase correlations)
NR-13 (Brazil) — documentation reference; LOGOS does not perform legal classification nor replace the construction code

Typical reference values

Quantity	Typical range	Note
Flash steam velocity limit	15.24 m/s (3000 ft/min)	FCI ST-108 value, calculator default and adjustable.
Liquid velocity in the discharge (when the liquid dominates the volume)	2 m/s	LOGOS design criterion, adjustable — not taken from a reference. It is also the default limit for the pump discharge in a pumped return.
Spirax Sarco, Module 14.3 (drain line before the trap)	1.5 m/s and 200 Pa/m up to 10 m; 1 m/s and 100 Pa/m above 10 m	They apply to the liquid line BEFORE the trap, which this calculator does not size. That is why they are not used as the discharge limit.
Load factor for trap selection	2	Market practice adopted as default; adjustable (≥ 1) to the company standard.
Back pressure per metre of lift after the trap	about 0.1 bar/m	Hot condensate column; calculated with the saturated liquid density at the destination pressure.
Default discharge line roughness	0.045 mm	Commercial steel; adjustable for existing lines.

Worked example

Heat-exchanger trap at 7 bar(g) discharging to a pressurized return at 0.5 bar(g), with a 4 m lift

Inputs

Condensate destination: Pressurized return -
Destination pressure: 0.5 bar(g)
Lift up to the return high point: 4 m
Single-phase losses of the discharge: 0.05 bar
Discharge line (Di × L): 35.04 × 30 mm × m
Nominal operation (trap pressure · load): 7 · 400 bar(g) · kg/h
Minimum operating pressure (pressure · load): 1.5 · 250 bar(g) · kg/h
Start-up (pressure · warm-up load): atmospheric (assumed) · 600 bar(g) · kg/h
Load factor: 2 (default) -
Manufacturer capacity curve: not entered -

Results

Flow in the discharge: Pressurized line with flash steam -
Flash steam at destination (fraction): 45.5 (11.37 %) kg/h
Minimum diameter by flash: 34.97 mm
Flash velocity in the line / limit: 15.18 / 15.24 m/s (line meets)
Start-up — back pressure · ΔP · load × factor: 0.92 · −0.92 · 1,200 bar(g) · bar · kg/h — no differential at start-up
Minimum pressure — back pressure · ΔP · load × factor: 0.92 · 0.58 · 500 bar(g) · bar · kg/h — capacity not evaluated
Nominal operation — back pressure · ΔP · load × factor: 0.92 · 6.08 · 800 bar(g) · bar · kg/h — capacity not evaluated
Two-phase ΔP in the discharge: not evaluated -
Worst trap condition: no differential at start-up -

Dropping from 7 to 0.5 bar(g), 11.37 % of the condensate flashes (45.5 kg/h of flash steam), but the liquid takes less than 1 % of the flash volume (ratio 0.0071). That is why the line is sized by the flash: the minimum is 34.97 mm and the 35.04 mm line meets it at the limit, with flash at 15.18 m/s for a 15.24 m/s limit. The back pressure is 0.92 bar(g) — 0.5 bar at the destination, plus the 4 m column, plus 0.05 bar of declared losses and the liquid-phase friction as a floor (0.0015 bar in nominal operation). At start-up, with the equipment at atmospheric pressure, the differential is −0.92 bar: the trap does not discharge, and the calculator recommends considering start-up drainage to a safe destination or a pump-trap. At minimum pressure and in nominal operation there is differential (0.58 and 6.08 bar), but capacity is not evaluated for two reasons: the manufacturer curve was not entered, and the line two-phase ΔP is left out because none of the three correlations (Friedel, Lockhart–Martinelli, Müller-Steinhagen & Heck) has a documented application range for this regime. They are calculated for information only — here they give between 0.09 and 0.22 bar — and even with the curve the result would not go beyond “not evaluated”, because the real differential is smaller than calculated. Note also that the trap table shows 40.1 kg/h of flash in nominal operation: there the flash is calculated at the 0.92 bar(g) back pressure, while the line uses the destination pressure, with a larger volume.

Run your own calculation Create a free account and get this for your own inputs in seconds.

Common mistakes

Entering the supply header pressure instead of the pressure upstream of the trap. In equipment under modulating control it is lower and falls at part load — that is where the differential is missing.
Leaving start-up out. With the equipment near atmospheric pressure and the warm-up load, the largest of all, a trap discharging to an elevated or pressurized return has no differential.
Sizing the trap discharge as a water line. Flash steam takes almost the whole section: the line is sized by its velocity, not by the condensate velocity.
Forgetting the lift after the trap. Every metre of rise adds about 0.1 bar to the back pressure.
Counting a downward run in the discharge as a back pressure reduction. A non-flooded discharge with flash has no continuous liquid column; the negative elevation does not help.
Including pipe friction in the declared single-phase losses when the discharge line is already entered. Pipe friction is already calculated; the declared losses must be fittings and equipment only (check valve, sight glass, strainer).
Approving the trap on catalogue capacity with an incomplete back pressure. Without the line two-phase ΔP, the real differential is smaller than calculated and the calculator does not issue “meets” on that basis.

Frequently asked questions

Why is the condensate line sized by flash steam and not by water?

Because flash steam takes almost the whole section. In the example, 11.37 % of the mass flashes, but the liquid volume is less than 1 % of the steam volume. The calculator follows FCI ST-108: area by flash velocity (3000 ft/min, 15.24 m/s by default) plus the liquid area fraction, with properties at the destination pressure, where the volume is largest.

How does the lift after the trap affect the back pressure?

Every metre the condensate rises after the trap adds about 0.1 bar (condensate column, ρ_l·g·Δz). A downward run only lowers the back pressure if there is a continuous liquid column — flooded line or no flash — and never below the vapour pressure of the liquid. In a non-flooded discharge with flash the calculator adopts Δz = 0 on the way down.

Why does the two-phase ΔP show as “not evaluated”?

The calculator only adds the two-phase ΔP to the back pressure when the correlation has a documented application range in the source. Friedel, Lockhart–Martinelli (Chisholm form) and Müller-Steinhagen & Heck are calculated, but today none has that range for condensate return, so the values show for information only and the ΔP is declared not evaluated instead of estimated. In that case the liquid-phase friction enters as a floor: stall and lack of capacity remain conclusive, but “meets” is not issued. Without the discharge line entered (with flash and a pressurized destination), the trap is also not evaluated.

Why check the trap at start-up and minimum pressure, not only in nominal operation?

Because the differential is usually missing outside nominal operation. At start-up the equipment is near atmospheric pressure (the assumption when the pressure is not entered) and the warm-up load is the largest. In modulating equipment the trap pressure falls at part load. ΔP ≤ 0 at start-up is a start-up warning (consider start-up drainage or a pump-trap); ΔP ≤ 0 in operation is stall (consider a pump-trap or reducing the back pressure).

Where do the trap capacity and the factor of 2 come from?

Capacity comes from the manufacturer curve (ΔP and kg/h pairs of the chosen model), interpolated at the differential of each condition; the minimum operating differential is also manufacturer data. Without the curve, capacity is “not evaluated” — the calculator does not invent catalogue data. The required load is the condensate load times the selection factor, 2 by default (market practice), adjustable. Outside the curve range, capacity is extrapolated conservatively and flagged.

Does the calculator size the pumped return and the pump NPSH?

In a pumped return the trap discharges into the receiver (vented, 0 bar(g), if the pressure is not entered), and the trap back pressure comes from the receiver, the lift and the losses up to it — not from the pump discharge pressure. The pump discharge is checked separately by the maximum pump flow, by velocity (2 m/s LOGOS default) and pressure drop. The pump NPSH is not calculated; the calculator only reports the subcooling at suction and warns to check the available NPSH.

Glossary

Flash steam: Steam formed when hot condensate drops from the trap pressure to a lower pressure; the fraction comes from the enthalpy balance between the two pressures.
Steam trap back pressure: Pressure at the trap outlet: destination pressure, plus the condensate column of the lift, plus the discharge line losses.
Differential at the trap (ΔP): Pressure upstream of the trap minus the back pressure. The trap only discharges with a positive ΔP.
Stall: Operating condition in which the pressure at the trap does not overcome the back pressure (ΔP ≤ 0): there is not enough differential to discharge through a conventional trap.
Pump-trap: Alternative pointed out by the calculator when there is not enough differential for a conventional trap, at start-up or in stall.
Flooded line: Discharge with a continuous liquid column. Only there does a downward run lower the back pressure, limited to the vapour pressure of the liquid.
Load factor: Multiplier of the condensate load for trap selection; 2 by default, adjustable.
Receiver: Pumped-return vessel into which the trap discharges; the pump delivers from there to the final destination.