Two most useful shape drawing tool in Inkscape is Rectangle and Circle. What if a single piece of more complicated shape is needed? First tool that comes into mind may be using drawing pen for Bezier Curves & Straight Lines, however, using Path Union Operation & Path Difference Operation would be the easier option. These operations can be found under Path menu, but it's pretty convenient to associate the operations to the shortcut key of Ctrl + Plus & Ctrl + Minus respectively. In most cases, engineer only needs to use these two "combining" and "cutting" actions to achieve the desired shape, so although the other path operations are more direct to the result but might be rarely used. But then again this greatly depends on personal preference.
Path Operations Menu.
Below is several demonstration of Path Operation steps in drawing a simple pump symbol like below.
A simple pump symbol to be drawn.
Step 1. Draw the desired shapes for pump body and highlight them.
Step 2. Hold down Ctrl key then press '+' to perform Path Union operation.
Step 3. Draw the legs by first drawing one big circle then place another smaller circle on top.
Step 4. Highlight two circles then hold down Ctrl key and press minus (-) to perform Path Difference operation.
Step 5. The result is the big circle has a cut out of small circle.
Step 6. The next step is to draw another rectangle and place it on top then highlight both of them.
Step 7. Doing another Path Difference operation will cut off the bottom part.
Step 8. Place the leg behind the pump body, then draw another circle to place on the pump body shall complete this simple demo.