Silly question, what's the purpose of the non returning valve?

What's the diameter of the table?

Hi all, need some inputs on autogate material. Was wondering which one is the cheapest and which one is the most expensive: powder-coated steel, galvanized steel, mild steel and stainless steel?

hi, anyone know the cost of replacing old roof tiles with new roof tiles (cement tiles) and changing the support structure from wood to steel for a roof that is about 20ft wide and 50 ft long? and also use aluminium foil as insulator?

in a dilemma now on wiring. one contractor quite cheaper, rm43 per downlight, inclusive of wiring, installation and cutting holes, another quoted rm68 but the cheaper one said will do 2 to 3 looping for the 13A power outlet.

but if don't do looping i think i need 3 DB cause 13A+HVAC points alone got 47 points. what's the maximum switch that each DB can take for single phase?