I'm trying to determine the amount of steam required to heat a water tank on our plant. How many pounds of 30 psig steam are needed to heat 1000 gallons of water from 70F to 160F?
and how do you calculate this?

I think that it's: [(pounds of water) * (change in temp)] / [total heat of evap. - latent heat of evap]

Pick-up a copy of Spirax Sarco's "Hook-Ups", and all will be revealed. If you suggest that you might buy something like a temperature control valve or steam trap, the local rep will likely just give you one.

You better hope the tank is not losing heat to atmosphere while you are heating the water. Might be a good idea to heat the tank as well as the water. Hint - for a vertical surface insulated with 1.5 inches magnesia the heat loss will be about 2 to 2.5 BTU/hr/sq ft/deg F . That's as long as the wind does not blow. TBP gave you a good hint.  

Assuming steam is saturated in both cases, and if steam and water are mixed, and the mix reaches a final temperature of 160^oF, the heat supplied by steam should be increased by the sensible heat from its saturation condensation temperature down to 160^oF.

In that case, the 150 psig-steam would supply a bit more heat per pound.

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