PID Controller to Turn: Couple questions on the code (pasted below)

I’ve written the following code. It’s a PID controller to have my robot turn 90 degrees left. I think I’m getting the hang of PID control. Couple questions I’ve got:

1. I need to set a cap for my integral term. What should I cap the integral at?
2. How can I use the integral term to correct steady state errors in this code? I don’t quite understand the function of the integral in this PID controller.
3. I understand the function of the derivative and proportion constants. I don’t quite understand how the integral term will correct steady state errors. Maybe there’s an issue with my code regarding how the integral of error is calculated.
4. I noticed that increasing kD (derivative constant) makes my robot oscillate more when getting closer to 90 degrees. Isn’t the derivative supposed to dampen oscillations?

  // PID Constants and Variables
  double kP=2;
  double kD=2;
  double kI=1;
  double error_prior=0;
  double integral_prior=0;

  turnSensorUpdate();
  double angle=((int32_t)turnAngle >> 16) * 360 >> 16;
  double error = 90-abs(angle);

  double integral=integral_prior+error*(10);
  double derivative = (error-error_prior)/10;
  integral = constrain(integral,-90,90);

  double turnSpeed=kP*error+kI*integral+kD*derivative;
  motors.setSpeeds(-turnSpeed, turnSpeed);

  error_prior - error;
  integral_prior = integral;
  delay(10);

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