Electrical – can i use a transformer to run an american steam cleaner ( 120v/60hz) on a 220v/50hz

Yes, you could buy and install a step-down transformer. However, heating appliances use a lot of power (an electric grill might use 900–1500 watts) and the corresponding transformer will likely be as much as the price of the appliance. Here are some which might do. Notice that a 1500 watt transformer weighs 18 pounds (8 kg)! The transformer should be rated at least twice the nominal load of the heating device to handle the cold start power curve.

If you are in an area with unstable voltage, a regulated transformer would provide some protection against under and over voltage. See these.

It is certainly better to buy an appliance which adapts to different voltages—which is easy with most electronic gizmos like computers and cell phone chargers—or is made specifically for 240 V.

The code requirement in most areas for the P&T relief plumbing is that it must be a material suitable for potable water plumbing - which in turn typically means it must allow a working pressure of 100psi at 180f. This might seem like an inadequate set of requirements for something intended to drain boiling water or steam, well over 180f, but the key term there is working - that is, it needs to remain durable when exposed to that pressure and temperature over long terms. The relief drain plumbing does not need to support pressures that high, and will only briefly be exposed to high temperatures so long term reliability is not required.

You may notice that PEX tubing is marked as complying with ASTM F877. ASTM F877 standards require that the tubing is able to endure 150psi at 210F for 30 days straight.