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[sgkim]

Jen,

Water hammering may arise if the partial pressure of the steam injected is higher than the vapor pressure of the steam at the temperature of hydrocarbon vapor(HC)/steam mixture.

The stem injected shall drop in pressure and slightly be superheated. Both sensible and latent heat of the steam will raise the temperature of the mixture. If the temperature of the HC/steam mixture drops below the saturation temperature of the steam, steam will condense and hammer.

To prevent the hammering, the HC should be heated only by the sensible heat of the superheated steam. Thus,

Q= M * Cp (Ts-To) = m*Cps(T-Ts) ----------------(1)

Q=heating duty,
M, m=molar flow rates of HC and steam respectively,
Cp,Cps=(average) molar specific heats of HC and superheated steam respectively.
To=temperature of HC before injection of steam
Ts=saturation point of steam at which steam condenses
T=temperature of superheated steam after injection

In the HC/steam mixture,

Ys= m/(M+m) = Ps/Pt, 1 = Y + Ys,

Y,Ys=mole fractions of HC and steam in the mixture,
Pt=total pressure of the mixture,
Ps=saturation press. at Ts or partial press at saturation.

Ts can be calculated from Ts=To+Q/(M*Cp), or from operting tempeature of the mixture, and T from the enthalpy balance of steam:

(T-Ts)=(H-hs)/Cps -----------------------------------(2)

H=Enthalpy of #200 or #600 steam
hs=Enthalpy of cendensate at saturation point Ts.

Then m ≥ Q/(H-hs) = M*Cp(Ts-To)/(H-hs)---------(3)

The flow rate m shall be supplied from #600 or #200 steam.
#600 steam has higher value of H (Ehthalpy) than #200 steam. So less amount of steam will be required if more superheated steam is used.

m1 and m2 are flows of #600 steam and #200 stem respectively
and are mutually dependent quantities, or m = m1 + m2.

You can get the idea from the equation (3) above.

Sefano/050802